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GUAYULE 

(PARTHENIUM ARGENTATUM GRAY) 

A RUBBER-PLANT OF THE CHIHUAHUAN DESERT 



BY 



FRANCIS ERNEST LLOYD 

Professor of Plant Physiology, Alabama Polytechnic Institute 




WASHINGTON, D. C. 
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON 

1911 



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PLATE 1 








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A. Guayule field with all plants above 40 cm. removed. Lomas of Sierra Zuluaga. 

B. Guayule field of maximum density, near Aplzolaya. 



GUAYULE 

(PARTHENIUM ARGENTATUM GRAY) 

A RUBBER-PLANT OF THE CHIHUAHUAN DESERT 



BY 

FRANCIS ERNEST LLOYD 

I) 

Professor of Plant Physiology, Alabama Polytechnic Institute 




WASHINGTON, D. C. 
Published by the Carnegie Institution of Washington 

1911 



y 



K^ 






CARNEGIE INSTITUTION OF WASHINGTON 
Publication No. 139 



Tv , 



Copies of this Book 
were first issued 

JUL27J911 



PRESS OF J. B. LIPPINCOTT COMPANY 
PHILADELPHIA, PA. 



PREFACE. 



In I go 7 the author of the present paper was engaged by the 
Continental-Mexican Rubber Company and the Intercontinental Rub- 
ber Company to organize investigations looking toward the successful 
cultivation of a Mexican desert rubber plant, the guayule {Parihenium 
argentatum Gray). Dr. Theodore Whittelsey and Dr. J. E. Kirkwood 
later became identified with this undertaking, the former as chemist, 
the latter as assistant botanist. The headquarters for the investiga- 
tions were established at the Hacienda de Cedros, Partido de Mazapil, 
Zacatecas, Mexico. It was not a matter for congratulation that, at the 
close of a year, the directors found it inadvisable, for financial reasons 
consequent on the panic of 1907, to continue the department of inves- 
tigation. By the courtesy of the company, however, the author carried 
on his studies for some three months beyond the termination of his 
business relations with it, and this period, falling during the growing 
season of 1908, brought to light many important facts. Still further 
observations of capital importance, in part on experiments begun in 
1907 and 1908, were made by the writer in April 1909, while represent- 
ing the United States Rubber Company, a commission which could not 
have been prosecuted without the kind concurrence of President C. C. 
Thach and a number of the WTiter's colleagues at the Alabama Polytech- 
nic Institute. As silence was not imposed by the United States Rubber 
Company, it has been possible to include these observations. 

No less than heart}' recognition is due also to Mr. W. H. Stay ton, 
formerly captain, U. S. Navy, sometime president of the Continental- 
Mexican Rubber Company, and now president of the Texas Rubber 
Company. It is stating an open secret to say that it was through the 
initiative and enthusiasm of this gentleman that the work of the inves- 
tigation was undertaken and would have been continued but for cir- 
cumstances beyond his control. Mr. Stayton has shown a liberal and 
scientific spirit, qualities not of necessity nor at all times associated. 

Thanks are due further to Prof. J. C. Arthur and Prof. W. G. Far- 
low for reports on various pathological matters; to Dr. M. T. Cook for 
contributing manuscript on the galls found on guayule; to Dr. A. D. 
Hopkins for a report on the guayule bark-borer; to Dr. L. O. Howard 
and Dr. J. G. Sanders for the identification of certain insects; and to Prof. 
B. L. Robinson for his courtesy in causing a photograph of the type speci- 
men of guayule to be made. Mr. Charles S. Ridgway has rendered sub- 
stantial aid in the preparation of certain figures. 

The drawing for figure 5 was supplied by Professor Arthur; the nega- 
tive of plate 2, fig. B, was made by Dr. W. E. Hinds; Professor Trelease 
furnished the illustration (fig. 4) and description of the Cedros sotol, and 
kindly made several other determinations; the negatives of plate 3 and 



iv Preface. 

plate 4, fig. A, were made by Mr. Victor Blanco. Dr. H. van der Linde 
obtained for me valuable material of irrigated plants from Caopas. 

Dr. Theo. Holm has afforded me the benefit of his criticism of the 
portion of this work treating of the anatomy, and has been good enough 
to examine inaccessible literature for me. Dr. W. E. Safford did a like 
service regarding a few pages in the first chapter. 

To Prof. W. L. Bray I am indebted for information about the Texas 
guayule fields, later verified by me personally ; and to my colleagues. Prof. 
C. L. Hare and Prof. J. P. C. Southall, for assistance in making chemical 
analyses and for mathematical formulae, respectively. 

With reference to the chapters which follow, no pretensions are 
made with regard to completeness. The exhaustive study of a single 
plant from all points of view might well be numbered among the labors 
of fable. The reader is asked also to remember that the study of but 
a single growing-period was possible. Much of the experimentation, 
therefore, was done, as it eventually turned out, during the most un- 
favorable season; but in the case of field experiments this was not 
entirely a misfortune. That the theoretical bearing of many observa- 
tions and more refined methods of making them are less attended to than 
the matter warrants has been due to the urgent necessit}^ of practical 
success. With these qualifications, the work may be regarded as a 
report on a unique opportunity, unhappily shortly terminated, to bring 
a hitherto feral desert plant under the subjugation of culture. That suc- 
cess may ultimately be attained is not an unreasonable nor an unwar- 
ranted expectation, for which statement the interested reader will find 
not a little evidence in what follows. 

Francis Ernest Lloyd. 
Alabama Polytechnic Institute, 
January 1910. 



TABLE OF CONTENTS. 

Page. 

Preface MI 

List of plates vu 

Chapter I. — Historical Account. 

Original discovery and description 3 

The vulgar name 4 

Primitive and later uses 5 

History of manufacture 7 

Methods of extraction ° 

The natural supply of shrub ^° 

Attempts at culture ^ ^ 

Chapter II. — The Environment. 

Geographical distribution ^3 

Altitudinal distribution ^3 

Climate ^4 

Rainfall ^4 

Air-temperatures ^| 

Soil-temperatures ^° 

Soil-moisture ^9 

Relation of rainfall and temperature to growth 20 

Relative humidity ^^ 

Topography and soil ^3 

Density of growth ^ 5 

Biotic relations 35 

Chapter III. — Description of the Guayule. 

Seed 46 

SeedUng 4» 

The mature plant 5° 

Root-system 5° 

Retonos 5^ 

Method of branching 54 

Biotypes 55 

Size 56 

Surface characters of the stem and method of determmmg age 57 

Field plants 57 

Irrigated plants 5° 

The leaves 5» 

The inflorescence and the flowering period 59 

The production of seed "° 

Chapter IV. — Reproduction. 

Methods of reproduction ^' 

Retonos, normal and induced 2o 

Seed ^^ 

Rate of reproduction and of growth. 75 

Rate of growth during germination 75 

Rate of growth in maturer plants beyond the seedlmg stage 79 

Rate of growth in terms of stem-length 79 

Rate of growth in earlier years after germination 79 

Rate of growth in medium-sized plants 81 

Rate of growth in irrigated plants °4 

Field plants °5 

Chapter V. — Anatomy and Histology. 

Root 90 

Primary structure . . . . ; 9° 

Secondary structure 9° 

Hypocotyl 9 

Primary structure 9° 

Secondary structure 99 

Later secondarv stntcture ^°^ 

v 



vi Table of Contents. 

Chapter V. — Anatomy and Histology — Continued. 

Page. 

Age and structure in the seedling 104 

Epicotyl 105 

The definitive stem 107 

Primary structure 107 

Secondary structure 109 

Origin of the medullary and cortical stereome no 

Annular structure 114 

The effect of abundant water upon anatomical structure 116 

Relative volumes of cortex and wood 117 

Effect of various amounts of water of irrigation 121 

Elifect of drought following irrigation 122 

Efifect of irrigation on the physical characters of the wood 122 

The peduncle 124 

The leaf 125 

Cotyledons 125 

Prophylls 126 

The definitive leaf 126 

Chapter VI. — The Resin-Canals in the Guayule. 

The canal-systems 165 

Primary canals in the root and hypocotyl 165 

Primary cortical canals 166 

Medullary canals 169 

The canals in the leaf 171 

Primary canals in branches 171 

Secondary canals in root, hypocotyl, and stem , 172 

Canals in the peduncle 172 

The canals in retonos 173 

The contents of the canals; their origin 174 

The role of resin 174 

Resin-content of guayule by analysis 175 

Chapter VII. — The Origin and Occurrence of Rubber. 

Methods 176 

General distribution of rubber in the plant 177 

The appearance of rubber in richly loaded tissues 179 

Behavior of peridermal divisions toward rubber 179 

The development of rubber in the cell 180 

Centers of secretion 181 

Rate of rubber secretion relative to growth 183 

Rubber-content by chemical methods 185 

Variation in relative amount of rubber in field plants 187 

Relation of rubber and resin, 188 

The significance of rubber 188 

Summary 190 

Chapter VIII. — Vegetative Reproduction. 

Induced root-regeneration 193 

Propagation by cuttings 195 

Chapter IX. — The Cultivation of Guayule. 

Forestal operations 199 

Present field operations 199 

Suggested rules of practice 200 

Harvesting period 202 

Reseeding barren ground 202 

Cultural operations 203 

Seed 203 

The raising of seedlings 203 

Irrigation ' 208 

Transplanting 209 

Harvesting cultivated guayule 210 

Catch crops 210 

Bibliography 211 



LIST OF PLATES. 



Plate i 



Facing 
page. 

A. Guayule field with all plants above 40 cm. removed. ^ 
Lomas of Sierra Zuluaga > Frontispiece 

B. Guayule field of maximum density, near Apizolaya J 

2. A. The type specimen of Parthenium argentatiim Gray \ 

B. Transverse section of a very old stem ( 

3. A. Upper floor in a guayule factory from which pebble-mills are 

charged 

B. Lower floor: discharge chutes and ditch from pebble-mills . . 

C. A battery of washing and sheeting machines J 

4. A. Stacks of guayule in bales. Continental-Mexican Rubber Com- 1 

pany's factory 1 g 

B. Experimental ground, with plants two years old from stocks. , 
Cedros J 

5. A. Station 2, Quadrats 5 and 6. Lomas of Sierra Zuluaga 1 

B. Station 3, Quadrat i, near Cedros. A good stand of mature V 24 
plants ) 

6. Plants from Quadrats 5 and 6, Station 2 24 

7. A. Quadrats (Station 12) with very dense growth. Apizolaya. . . | 
B. The same, the guayule removed J 

8. A. Narrow type of guayule "I ^5 

B. Spreading type of guayule / 

9. A. The root-system of guayule | 

B. Groups of plants which started as retonos Y 48 

C. A strongly monopodial retono J 

An exceptionally tall (130 cm.) individual. Caopas 52 

A. A widely spreading (130 cm.) plant of guayule \ 

B. A large plant of the usual habit. Apizolaya J 

[2. A biotype of guayule. The winter condition on the left 52 

[3. A-C. Seedlings of typical and atypical guayule \ g 

D. Seedlings and mature plants of these biotypes j 

[4. A. An irrigated plant, from a small stock, at the height of flowering 1 .^ 

B. "Hembra" (on the left) and "macho" (on the right) guayule. J ^ 
:5. A. Induced retonos on a tap-root. One season's growth \ ^^ 

B. Induced retoiios on a lateral root. One season's growth I 

[6. A-C. New growths after pollarding • \ gg 

D. Seedlings in limestone soil; E, in "garden" soil i 

7. A. Minimum, average, and maximum seedlings. (Station 2, quad-) 

rat4) [ 68 

B. Irrigated plant, two years old from a stock. Cedros J 

8. Seedlings growing in different soils 72 

J 9. Seedlings growing in different soils 7 2 

20. A. (i) Root-cutting; (2-4) sectorial root-stem cuttings (Exp. 146). 1 
B. Seedlings grown in different soils / 

21. A. A branch, one year's growth under irrigation 1 g 

B. A branch in the height of flowering, second season J 

22. Anatomical and histological details, figs, i to 16 128 

23. Anatomical and histological details, figs, i to 9 130 

24. Anatomical and histological details, figs, i to 13 132 

25. Anatomical and histological details, figs, i to 10 134 

26. Anatomical and histological details, figs, i to 13 136 

27. Anatomical and histological details, figs, i to 10 138 

28. Anatomical and histological details, figs, i to 5 140 

29. Anatomical and histological details, figs, i to 6 142 

30. Anatomical and histological details, figs, i to 1 1 144 

31. Anatomical and histological details, figs, i to 14 146 

vii 



viii List of Plates . 

Facing 
page. 
Plate 32. Anatomical and histological details, figs, i to 7 148 

33. Anatomical and histological details, figs. 1 to 10 150 

34. Anatomical and histological details, figs, itog 152 

35. Anatomical and histological details, figs, i to 15 154 

36. Anatomical and histological details, figs, i to 8 i 56 

37. Anatomical and histological details, figs. ito8 158 

38. Anatomical and histological details, figs, i to 18 160 

39. Anatomical and histological details, figs, i to 8 162 

40. I. Rubber in canal-cells, nearby cortex and inner ray-cells 

2. Older root. More rubber in rays 

3. Root 2 mm. diameter 

4. Parenchyma ray from fig. 2 

5. Upper part of hypocotyl, same age as fig. i 

6. Longitudinal section through old wood [- 172 

7. Longitudinal section through mature leptome parenchyma, with 

a few parenchyma-ray cells 

8. Leptome ; elongated elements 

9. Companion cells and sieve-tubes. No rubber in younger leptome 

on the left j 

41. 1,2. Cortex, stem of field plant with maximum rubber-content. . ] 

3. Cortex of a 20-year-old stem j 

4. Root ; rapidly grown seedling, 2 months old. Rubber in granules. [- 172 

5. Rubber in process of accumulation in an irrigated plant i 

6. Primary resin-canal J 

42. I. Apex of terminal twig of 1908, field plant "| 

2 . Near base of same | 

3. Pseudotylosis with rubber in the cells 

4. Leptome, field plant 

5. Pith of a field stem 10 mm. diameter \ 176 

6. An old leaf-trace 

7. Outer cortex of a field stem 

8. Outer edge of cortex and inner zone of cork derived from coUen- | 

chyma J 

43. I. Base of 1908 growth, August. Cedros, irrigated 

2. Growth of 1908 in April 1909. Cedros, irrigated 

3. Cortex, 2-year-old stem. Caopas, irrigated \ 192 

4. Pith of same plant . | 

5. Epidermis and palisade of an old leaf, field plant J 

44. A. Irrigated plant, 2 years old. Basal branches which have rooted 1 

are spread apart V 192 

B. Mariola showing the same behavior, normal in this species. ... J 

45. A. Flat filled with paper tubes, I square inch in transverse section. ] 

B. Flat with 4-square-inch tubes 

C. Exp. 141 (3), I -inch tubes; very poor growth. Exp. 143, 4-inch }- 204 

tubes 

D. The same, seedlings well grown 

46. A. Seedlings from experiments indicated ) „ 

B. Irrigated plant (Caopas) with a retoiio / ^° 



GUAYULE (PARTHENIUM ARGENTATUM GRAY): 

A RUBBER PLANT OF THE CHIHUAHUAN DESERT. 



By 

FRANCIS ERNEST LLOYD, 
Professor of Plant Physiology, Alabama Polytechnic Institute. 



CHAPTER I. 
HISTORICAL ACCOUNT. 

Since about the middle of the last century, after the epoch-making 
discovery of Charles Goodyear was made, the demand for crude rubber 
has been steadily increasing. This demand was for a long period satis- 
fied by the products harvested from the tropical forests of the Old 
and New Worlds by natives whose methods are resulting in a gradual 
depletion of the natural supply. This, in turn, has stimulated research 
in three directions : toward obtaining a synthetic rubber, the ambition 
of the chemist; toward discovering other rubber-producing plants, for 
which search has been made into the farthest reaches of the tropical 
forests of the world; and, finally, in the direction of the culture of the 
various plants which before had been, in their feral condition, the source 
of the much-desired material. Whatever the promise of the chemist 
may be, there appears to be no abatement of interest at present in the 
culture of those better-known trees which have been found to adapt 
themselves to the hand of man, or in the discovery of hitherto unknown 
rubber plants. Each new announcement, however vague the authority 
may be, that a new rubber plant has been found, is hailed with precipi- 
tous interest; and one that is well founded is soon followed by a period 
of exploitation scarcely less fevered than on the finding of new gold- 
bearing fields. When, a very few years ago, it became more generally 
known that the plant commonly known as the guayule, and containing 
an economically valuable amount of rubber, grew in abundance in the 
desert country of northern Mexico, the vegetation of the adjacent arid 
areas underwent minute examination in the hope of finding either this 
or other similarly valuable plants, and even at the present moment the 
excitement has not died away. 

The mere fact, however, that a plant indigenous to the desert 
should be found to be of enough value to set in motion large business 
operations involving millions of capital, based on the amount of the 
raw material in sight, is sufficient to awaken definite interest. The 
economic value of the desert is changed, and possibilities for the devel- 
opment of wealth in a supposedly barren country take on new dimen- 
sions. This has occurred, in point of fact, as a direct result of the dis- 
covery that the plant guayule produces in the neighborhood of lo per 
cent of its weight of "bone-dry" marketable rubber. With the eco- 
nomic history, bionomics, structure, and micro-chemistry of this plant the 
present essay has to deal. 

ORIGINAL DISCOVERY AND DESCRIPTION. 

The guayule was first discovered by J. M. Bigelow, M.D.,in 1852, 
while attached to the Mexican Boundary Survey, " near Escondido Creek, 
Texas." It was first described by Professor Asa Gray some years later. 
His original description was based upon the type specimen, which is now 

3 



4 Guayule. 

in the Gray Herbarium of Harvard University. A reproduction of this 
specimen is here given (plate 2, fig. A). The name in the right-hand comer 
is in the writing of Professor Gray. The label is Bigelow's field label. Fol- 
lowing is the description published in the "Botany of the Boundary," 
p. 86, 1859: 

Parthenium argentatum (sp. nov. ) : fruticosum, pube brevi appressima 
sericeo-incanum ; foliis spathulato-lanceolatis oblongisve in petiolum longe attenu- 
atis parce dentatis seu laciniatis sub-triplinerviis ; ramulis fioridis elongatis nudis 
oligocephalis; involucri squamis obtusissimis ; acheniis sericeis; pappo e paleis 
2 membranaceis lanceolatis. — Near Escondido Creek, Texas, in rocky places, 
Sept., 1852; Dr. Bigelow. — A well-marked species, connecting the sections Argy- 
rochasta and Parthenichaeta ; the leaves and branches whitened with a very fine 
and close silk-silvery pubescence, which appears to be wholly or nearly persistent. 
Leaves one to two inches long, including the tapering base and petiole; 2 to 5 
lines wide, mostly acute, scarcely veined, beset on each margin with from one to 
three salient teeth, or sharp lobes. Flowering branchlets slender, 4 to 8 inches 
long, nearly leafless and peduncle-like, bearing 3 to 7 sub-sessile heads (as large 
as those of P. incanum) in a cluster. Exterior scales of the involucre short, orbic- 
ular-ovate; the inner orbicular, scarious-membranaceous. Paleae of the pappus 
lanceolate or oblong-lanceolate, rather narrower and less obtuse than in P. hyster- 
ophorus, puberulent, the inner edge more or less adnate to the base of the broadly 
obovate and cucuUate emarginate ligule. ' (Fig. 9.) 

As will be seen, the crowding of the heads to form a "cluster" de- 
pends upon external conditions. In a later description published by 
Gray in the "Synoptical Flora," ^ we find the first hint of the peculiarity 
which later brought it into economic prominence. This description is 
as follows: 

P. argentatum Gray, Suffrutescent, a foot high, silvery-canescent with close 
tomentum; branches erect, rather leafless above, bearing comparatively large and 
few heads (of 2 lines in diameter) ; leaves lanceolate to spatulate in outline, some 
entire or incisely 2-3 toothed, the larger incisely pinnatified into 2 to 7 acute 
lateral lobes; pappus a pair of lanceolate chaffy awns (Bot. Mex. Bound., 86; 
Southwest border of Texas, Bigelow; Adj. Mex., Parry, Palmer; produces a gum 
or resin in Mexico). 

THE VULGAR NAME. 

The name ^ "guayule " is properly applied only to Parthenium argen- 
tatum Grav. On account, however, of a superficial resemblance it has to 
certain other plants, especially because of similarities in size and in the 
gray color (so often seen in the desert) of the foliage, these have been 
wrongly called by the same name.* The mariola (P. incanum H. B. K., 
plate 44, fig. B), a closely related species, is one of these; and its very 
general association with the guayule proper has led to much error in 
estimating acreage of guayule. It is of interest in this connection to note 
that the mariola is known to the peon, in some parts at any rate, as 
"hembra de guayule," ^ apparently because of the very constant associ- 

*Gray, in Torrey, Botany of the Boundary, U.S. and Mex. Boundary Surv., 
p. 86, 1859. 

^Synoptical Flora of North America, vol. i, pt. 2, p. 245, 1886. 

'Investigated by Endlich, 1905. 

*The name is also applied to Crysactinia mexicana Gray, and more recently 
also to Euphorbia misera, material of which was sent to Dr. J. N. Rose, of the 
U. S. National Herbarium, from southern California, on the supposition that it 
contained rubber. 

^The female guayule. 




At?' 






^ 



/-^>M</l^UHv ^»^«t,^<i/-t<-^^ j 



A. The type specimen of Parthenium argentatum Gray. 

B. Transverse section of a very old stem. 



Historical Account. 5 

ation of the two species, and because of the belief that this association 
is in some way necessary to the production of seed. Other species of 
the genus, some of which are annuals, have also received the name guay- 
ule, while a plant of the vSonoran Desert (Sonora and southern Arizona), 
Encelia farinosa, is not only mistaken to-day for guayule but is believed 
by many to contain rubber. The amount, if present at all, is so insig- 
nificant that it would certainly not repay consideration from a com- 
mercial point of view. 

The guayule is known also as "yerba de hule" in the region of 
Pasaje, Durango. and simply as "hule" in some parts of Zacatecas and 
of Chihuahua. An alternative spelling " yule " (which occurs incorrectly 
as "Hule" in "guallule") is used in some parts of San Luis Potosi. The 
name xihuite * occurs in northern Zacatecas and "about Saltillo"; 
copallin and afinador are other less-used designations. But the name 
"guayule" thus spelled is in the ascendant and will in all probability 
replace other names. Its derivation, in common with other Mexicanisms, 
has speculative interest. Seler ' would refer it to quahu (wood, tree, or 
forest) and olli (rubber, Sp. hule), evidently believing it to be of Aztec 
origin. This etymology finds support in the aboriginal term ulequahuitl, 
said by Sahagiin (1529) and Augustin Torquemada (161 5) to be applied 
to a latex tree (probably Castilloa) producing ulli, a dark resin which 
becomes very elastic (Jumelie, 1903). By inversion, we have quahu+ule. 
The suggestion that the derivation is from the Castilian hay (there is) 
and the Aztec olli, from which we therefore have hayolli, which becomes 
hayule and so guayule, can not be seriously entertained. 

PRIMITIVE AND LATER USES. 

Contact with the country peon of Mexico reveals a great deal of 
resourcefulness in the use of many plants. In out of the way places a 
game is played with a small, very resilient ball, not purchased in the 
market. It proves on examination to be of very pure rubber, obtained 
by communal mastication of the bark of the guayule. Altamirano 
(1906) tells us that country boys obtain rubber in a similar manner also 
from "tatanini," a name applied, in Queretaro, to Parthenium incanum 
and to P. lyratum. This custom dates back with fair certainty to the 
middle of the eighteenth century, having been noted by a Jesuit, one 
Negrete.^ 

Mr. W. H. Stay ton, formerly captain in the U. S. Navy, w^hen on 
duty in the Gulf of California, observed the Yaqui Indians ashore playing 
a game with a ball about twice the diameter of a baseball. The game 
consisted in throwing the ball from hip to hip. It is not unlikely that 
the ball was made of guayule rubber, which could have been obtained 
from the countrv east of the Sierra Madre. or even of rubber from tatanini, 



' From the Nahuatl xihuitl, weed. This spelling is given by Endlich (loc. cit.). 
"Jihuite" is given in Zacatecas. "Gihuete" occurs in a legal instrument drawn 
up at Matamoras, Coahuila, under date of March 9, 1905, in which also "hule" is 
given as designating guayule. 

^ Endlich, loc. cit. 

^According to Juan 'Fritz, fide Endlich, 1906. 



Guayule. 

mariola, or other plant. The possibility that it came from the South is, 
however, not excluded. Peter Martyr (1569; published in 1830), Saha- 
gun (1529), and Herrera (1492-15 26) all speak of balls made of rubber 
made from latex trees.' 

There can therefore be little doubt that, in common with the manu- 
facture of mescal, extraction of fibers, and like primitive industries, the 
making of rubber balls from the guayule, just as from latex plants, 
antedates the invasion of Mexico by the Spaniard. It may be mentioned 
in passing that the method of extracting the rubber as above noted is 
analogous to the only widely used modem method of obtaining the crude 
rubber on a large scale, namely, by a purely mechanical process. The 
rationale of this will be seen beyond. In this connection a recent dis- 
covery of a piece of rubber which is undoubtedly of ancient origin on an 
old aboriginal village-site in Arizona is of peculiar interest. Of this 
discovery the following account is furnished me by Prof. R. H. Forbes: 

The lump of rubber, a portion of which I recently handed you, was found in 
December (or thereabouts), 1909, at the west end of the Santa Cruz Reservoir and 
Land Company's dam, 14 miles west of Sasco, Ariz. Mr. C. O. Austin, who was 
present, states that this ball of rubber was contained in a small oUa with articles of 
stone belonging to the older prehistoric ruins of this country. The find was made 
at about 3 feet below the general surface which was formed by the off-wash of an 
adjacent low mountain. No traces of houses on the present level of the land, 
according to Mr. Austin, were visible. One other ball of rubber was found here, 
and is now in Col. W. C. Greene's collection at Cananea. I regard this find as 
genuine, as Mr. Austin is familiar with Salt River Valley ruins and his statements 
are confirmed by others. 

Microscopic examination of the specimen to which Professor Forbes 
refers throws doubt on the view that it is guayule rubber, but a final 
statement can not at present be made. 

A record of this kind would be incomplete without reference to the 
use of guayule as a fuel. On account of its resin content, the plant 
burns with a fierce, smoky flame, after the fashion of "fat pine;" so 
that whenever it was available it was invariably used as a fuel for the 
crude Mexican adobe smelters, ruins of which are frequently seen in the 
mining districts. In this way thousands of acres have been depleted 
of their guayule, a wasteful process which was quickly stopped when 
the value of the plant became known. It can scarcely be doubted that 
many peculiarities of local distribution within restricted regions are due 
to the pulling of the guayule for fuel. Thus a large smelter and a num- 
ber of roasting furnaces were in operation at Cedros,^ the head frq,ction 
of the hacienda of that name lying to the west of Mazapil, for a term 
of years, and this circumstance is often referred to by the peons to 
explain the absence of guayule in places where it would naturally be 
expected. The case is analogous to the use of walnut for fuel and fence- 
rails in the early days in the eastern United States. 

' Jumelle, 1903, quotes these authors at length. 

'According to Juan Robles, whose duty it was, in 1856, to weigh the shrub 
as it came into the fundicion at Cedros, guayule was paid for at the rate of 18 
centavos per carga (6 arrobas = 7o kilos), or about 17 pounds for i cent (gold)! 
The women on Cedros burned guayule in their bread ovens as late as 1894 {fide G. R. 
Fleming). Guayule shrub now fetches 150 pesos the ton. 



Historical Account. 7 

HISTORY OF MANUFACTURE. 

Public attention was drawn to guayule rubber,' apparently for the 
first time in 1876, by an exhibition sent from Durango to the Centennial 
Exposition at Philadelphia (Pearson, 1907). In the same year, accord- 
ing to the Mexican Herald, the Natural History Society of Mexico took up 
the study of the plant and reported the presence of rubber of good 
quality (Delafond, 1908). 

The first move toward the utilization of guayule rubber other than 
by the natives appears to have been made in 1888, when a company, 
the name of which is unknown to me, but probably the Mechanical Rubber 
Co., of Passaic, New Jersey, sent a special agent to Mexico with instruc- 
tions to "obtain a large quantity " of "rubber-bark," "from which it was 
proposed to extract the rubber by a process of grinding and washing." 
According to the account, the agent seems not to have clearly understood 
his instructions, and shipped to New York 100,000 pounds of the entire 
shrub! The company in question did not relish paying the freight on 
the wood, and this item of expense deterred further investigation. 
However, the shrub was decorticated, the bark and twigs ground up 
finely, and "immersed in hot water * * * finally coagulating the 
rubber into one mass. ' ' The result was an extraction of 1 8 per cent rubber 
(the wood of course not entering into the count), the quality of which 
was regarded as equal "to the best grade of Centrals," and a specimen 
was reported^ to have been in good condition in 1895. There seems to be 
little doubt that the "rubber-bark" referred to in the preceding para- 
graph was guayule, though ignorance of the identification was confessed. 
However, the material was collected at Hot Springs (Aguas Calientes) , 
Chihuahua, and was referred to in a letter by the local agent, who under- 
took the collection, as "hule."^ 

In this same year, 1888 or thereabout, a Mr. Herbert Wilson sent a 
sample of the rubber to England for analysis, and at about this time 
also Herr Juan Fritz employed a number of peons to chew out a suffi- 
cient amount of the raw material for examination, and this he sent 
for study to a German chemist, whose report was a practical condemna- 
tion of the rubber as an article of commerce. 

Shipments of crude shrub made to Hamburg in 1900 were treated 
with caustic soda and small amounts of rubber thus recovered were 
placed on the market. In the following year 25 or 30 pounds of guayule 
rubber were sent to the market from a laboratory which had been estab- 
lished by Germans at San Luis Potosi. The earliest eflforts seem to have 
centered here, so that San Luis Potosi may be regarded as the birthplace 
of the industry. 

The laboratory experience at San Luis Potosi led in 1902 to the 
establishment of a factory at Jimulco, by Adolf Marx, representing the 

» I have been unable to obtain a transcript from the original records. An 
anonymous writer in the India Rubber World, April 10, 1895, refers to this exhibit 
as rubber from "a native plant of the genus Cynanchum, of the natural order 
Asclepiadaceae, according to Mr. Fernando Altamirano." 

* In the India Rubber World, 10 : April, 1895: "Extraction of rubber from 
minor plants" (unsigned), upon which I base the account in this paragraph. 

' India Rubber World, loc. cit. 



8 Guayule. 

Compania Explotadora de Caucho Mexicano, from which factory rubber 
was put on the market for the first time in 1905. In 1902, also, certain 
American capitalists financed an expensive but eventually successful 
series of experiments which led to the successful extraction of the crude 
rubber by a mechanical process (devised by Mr. Wm. A. Lawrence), and 
two years later, in 1904, the first lot of rubber thus prepared was taken 
by the Manhattan Rubber Company. On December 25, 1904, 50 pounds 
of crude rubber, extracted by means of the now successfully adapted 
pebble-mill, were shipped to the United States, over half of the amount 
being purchased by the Manhattan Rubber Company. Then followed 
the building of a large factory at Torreon by the Continental-Mexican 
Rubber Company (plate 3, plate 4, fig. A), in which the results of the 
earlier experiments were used. This event marked the beginning of 
commercial success in the extraction of rubber from the guayule shrub 
by the mechanical method, which has superseded all others, and it 
should be said that this phase in the development of the industry is 
almost entirely due to American initiative and ingenuity. 

From 1905 on a large advance in the outlay of capital followed, and 
extracting plants of various sizes were established in San Luis Potosi, 
Saltillo, Monterey, and Gomez Palacio, as well as at Torreon and Jimulco. 

Manufacturing enterprise has lately brought the guayule industry 
into Texas. On September i, 1909, a factory ' at Marathon, Texas, in 
the heart of the guayule area of that State, began operations under 
the Texas Rubber Company. But it should be added that the manu- 
facture of guayule rubber had already to some extent been carried on 
in the United States and abroad. The extent of this phase of the indus- 
try is indicated in the total exportation of crude shrub from Mexico, 
the statistics for which are given on p. 11.- 

At the present writing, according to Mr. Henr}^ C. Pearson,^ the out- 
lay of American capital alone in Mexico amounts to $30,000,000. 

METHODS OF EXTRACTION. 

A brief statement of the principal features in the methods of extrac- 
tion of rubber from guayule will be of interest here, especially as they 
differ widely from nearly all hitherto-used methods of preparing crude 
rubber from latex plants.^ It must be understood that the rationale of 
the processes lies in the fact that the rubber exists as such in the cells of 
the plant, and will not escape by bleeding. 

The material must, then, either be dissolved out, after preliminary 
grinding, by suitable chemical agents, or must be agglomerated mechan- 
ically, either with or without the assistance of a substance (caustic soda) 
which will attack the cell wall. The chemical method is used successfully, 
it is understood, at Akron, Ohio, where an excellent brand of guayule 



* Previously built and operated for a short time by the Big Bend Rubber Co. 

^ We now read that the Japanese have entered the market, and are buying 
shrub (Dec, 1909). 

^ India Rubber World, 40 : 383, August i, 1909. 

^African "grass-rubber," however, is obtained in a crude way, but purely 
mechanically, from species of Landolphia (Jumelle, 1903). 



PLATE 3 





A. Upper floor in a guayule factory from which pebble mills are charged. 

B. Lower floor: discharge chutes and ditch from pebble mills. 

C. A battery of washing and sheeting machines. 



IS" 



W W W W! ill ! Wf! !!! ! ?JH !! ! !! ! ! ! ! !!H big Hi! HH Ul tlili W!i iiii iit i38i S! 8S I — 



w4t 



i 










-"^^^^ 



A. Stacks of guayule in bales. Continental-Mexican Rubber Co. 

B. Experimental ground, with plants two years old, from stocks. Cedros. 



Historical Account. 9 

rubber is produced. Although the principles involved are well known, the 
precise steps are preserved secret. The process, which is based on meth- 
ods of organic analysis, is not widely used, and only a small part of the 
total manufacture is carried on in this way. 

Of greater interest, not only in itself, but for the future economic 
development of the rubber industry, is the mechanical method. This 
may be described only in its broader outlines, since the steps employed 
by various manufacturers are changed from time to time as experience 
indicates. 

The shrub is first washed so as to free it from dust and other foreign 
matters which affect the specific gravity of the " worm " rubber by becom- 
ing attached to the agglomerated particles. It is then passed between 
rolls which comminute it while it is being sprinkled with water. ^ The 
rolls used have been supplied with knives, or have been adapted to 
pulverize the material, or, as now used, the shrub may be run twice be- 
tween corrugated rollers, running differentially, for the sake of even and 
fairly fine grinding. The resulting mass is then placed in a pebble-mill, 
which is a short cylinder of steel, lined with Belgian flint bricks, such as 
is used for grinding cement, paint, charcoal, and the like (plate 3, figs. 
A, B). The grinding is accomplished by means of Norwegian or Medi- 
terranean flint shore-pebbles. - 

The pebble-mill charge consists of one-third its volume of pebbles, 
one-half of water, together with 6 to 8 bushels of shrub. The mill is 
revolved at the rate of about 30 times a minute for a period lasting 90 
minutes to 2 hours, at the expiration of which time there results a finely 
ground pulp consisting of minute agglomerations of rubber mixed with 
fine particles of bagasse. This is separated from the dirty water in which 
it was ground and is then run into tanks, where a skimming process sepa- 
rates the rubber, which floats, from the bagasse, which sinks. A part 
of the bagasse, however, does not sink at this time, namely, that con- 
sisting of flakes of light yellow cork. 

Nor is the rubber free from particles of wood fiber, imprisoning more 
or less air, and this interferes with the complete separation of rubber and 
bagasse. The complete water-logging of the bagasse may be attained 
by means of a compressor, in w^hich the skimmed rubber, with its adhe- 
rent fiber, is subjected under water to a pressure of about 225 pounds 
for a period of 15 minutes to 2 hours, according to the kind of shrub 
being treated. Subsequent treatment in a beater-washer, an elliptical 
tank, supplied with a paddle-wheel of half its transverse diameter, 
prepares for the final separation of rubber and bagasse in settling-tanks. 

' It has been suggested (Whittelsey, 1908) that decortication, previous to 
grinding, would be an economy. It is interesting to recall that this was done — ■ 
on an experimental scale, albeit a generous one — in 1895 (India Rubber World, 
April 1895). 

An alternative method, recentlj^ proposed by Chute and Randel (India Rubber 
World, vol. 42, p. 360, 1910), consists in grinding the shrub dry and then deresinat- 
ing (the solvent to be recovered by distillation). The ground shrub, now supposedly 
free from resin, is then treated as here described, beginning with the pebble-mill. 

' The internal structure of this mill has been the subject of numerous patents. 
Thus, steel balls, associated with various forms of projections from the interior 
surface of the cylinder, have been used, but without supplanting the "pebble-mill." 



10 Guayule. 

An alternative treatment consists in allowing the washed rubber 
from the first skimming-tank following the pebble-mill to soak for a week 
in settling-tanks, during which time the bagasse becomes water-logged 
and sinks. The soaking is probably of value also in separating from the 
rubber certain substances, probably enzymatic in character, which other- 
wise would contribute to the earlier breaking down of the rubber. 

The clean rubber is now passed between corrugated and smooth 
rolls for the purpose of washing and sheeting (plate 3, fig. C), when the 
product is ready to be put on the market. Unless further treatment 
ensues, the rubber thus prepared contains about 25 per cent moisture, 
together with a proportion of resin. ^ 

Other special steps in treatment are applied to the separation of 
rubber from bagasse, or in preparing special grades. For example, 
boiling the skimmed rubber in a i to 2 per cent solution of caustic soda 
has been used as an aid in the separation of rubber and fiber, and for 
partial deresination by the saponification of the resin acids. By this 
means the amount of resin, 25 per cent, usually present, may be reduced 
to 17 or 18 per cent.^ Other modifications in treatment are necessitated 
by the condition of the plants when treatment is begun. Old, weathered 
and dried-out shrub is not worked with the same ease nor with the same 
result as fresh, while a certain amount of seasoning is an advantage. Con- 
siderable losses have been entailed by storing guayule in the yard exposed 
to the sun (plate 4, fig. A), as may be imagined if a million dollars' worth 
of shrub is handled in this way, even though the amount of deterioration 
is small. This loss is now avoided by placing the shrub in storehouses. 

THE NATURAL SUPPLY OF SHRUB. 

With such large interests at stake, it soon became a matter of 
moment to determine the relation of supply of the shrub to the manu- 
facture, as to total supply in sight, as to its rate of reproduction under 
natural conditions, and as to the possibility of its cultivation. 

The first of these questions was naturally the first to be raised, and 
many attempts have presumably been made to find an answer. The 
earliest, and, so far as I am aware, the only published calculation was made 
by Endlich {loc. «'/.), who assumed an average amount of half a ton per 
hectare in virgin fields. The total area of the general guayule region 
being taken as 75,000 square kilometers, and assuming that only one- 
tenth of this carries the shrub, Endlich arrived at the sum of 375,000 
tons total supply in Mexico, which, at the rate of 7 to 10 per cent of 
rubber, represents 26,250 to 37,500 tons of rubber. This estimate was 
probably quite conservative, as indicated by calculations based upon 
official reports brought together in the India Rubber World. 

Using the probable corrections for exports of crude rubber other 
than guayule, this publication gives the total imports of guayule rubber 

' Whittelsey, 1909. 

^ At this writing an announcement is made (Guayule Rubber by a New Proc- 
ess, India Rubber World, December, 1909) that a method ("physico-mechanical" 
— sic.) has been patented whereby crude rubber, after treatment, has the com- 
position: "Pure caoutchouc, 88 per cent; resin, 7 per cent; water, 5 per cent." 



Historical Account. 11 

into the United States from June 30, 1905, to June 30, 1909, as 32,010,820 
pounds. This being about 80 per cent of the total export, using the 
data for 1 906-1 908 ' as a basis, we have a total exportation of crude 
guayule rubber for four years of 40,013,525 pounds, which amount to 
20,000 long tons in round numbers, representing, on the basis of 7 per 
cent extraction of rubber with 25 per cent moisture (5.25 per cent dry 
rubber), shrub, 286,000 tons; and on the basis of 15 per cent extraction 
of rubber with 25 per cent moisture (11.25 per cent dry rubber), shrub, 
132,000 tons. 

The last two sums give us the highly probable extremes between 
which the tonnage of shrub represented by crude-rubber exports falls. 
To the amount must be added the amount of shrub exported, for which 
figures for two and a half years are available, namely, 2745 tons. We 
have, therefore, the limits of 288,745 tons and 134,745 tons. 

That the larger amount of shrub is nearer the true amount taken 
appears to be the case, since the extraction of rubber with 25 per cent 
moisture has only recently reached 15 per cent, and this is probably 
not attained by manufacturers in general. For a long time it fell below 
10 per cent, so that an average extraction of 8 to 10 per cent of rubber 
(25 per cent moisture) is probably near the truth. This would represent, 
on the 8 per cent basis, 252,745 tons shrub; on the 10 per cent basis, 
202,745 tons shrub. 

It is therefore probable that in the neighborhood of 225,000 tons 
of shrub were disposed of up to June 1909. This, according to Endlich, 
would be somewhat over half the total original available supply. This 
estimate agrees with that of some interested informed persons who hold 
that one-half of the original supply is used. But estimates carefully made 
for business purposes show that there were at this time at least 200,000 
tons still available. Of this amount, I myself have seen at least 100,000 
tons in a comparatively restricted area on three estates. 

Allowing for guayule still remaining on fields which have been gone 
over, and which in certain well-known cases is in considerable amount, 
it seems not improbable that the total original amount reached 500,000 
tons. The amount in Texas in the Big Bend country is not known and 
must therefore be left out of account, but without it it does not seem 
probable that the total amount of virgin shrub is sufficient to last more 
than four to six years at the present rate of consumption.^ It is likely 
that the smaller concerns will be closed out, so that, with a reasonably 
restricted output, the supply may be made to last six to eight years, 
which is the period during which the solution of the cultivation of the 
plant must be compassed if it is to keep the industry on its feet. 

1 India Rubber World, September 1909. For 1906 to 1908 the total crude 
rubber exports were 22,693,489 pounds, while our total imports were 17,917,342 
pounds. 

' The recent high prices paid for crude rubbers have stimulated the manu- 
facture of guayule rubber, which has brought as much as $1.25 per pound. The 
imports into the United States for the year ending June 19 10 were, approximately, 
10,000 long tons. On the basis stated above, this quantity represents something 
between 66,000 and 145,000 tons of shrub, but, in view of the improved methods, 
the smaller tigure lies nearer the truth. If we assume a 12 per cent extraction, we 
get 83,300 tons of shrub used in the year. 



12 Guayule. 

As in all commercial enterprises depending upon the rate of growth 
of the raw material, and more notably of lumber trees, the methods 
were and still are conducted without relation to the future. When, 
however, the capitalist began to see that nature had set a definite limit 
to the rate of supply, it became a matter of moment to determine what 
could be done to meet the demand. The method of obtaining the shrub, 
when not owned outright, is by contract between the companies and the 
hacendados whose lands support a growth of the desired plant. These 
gentlemen at first signed contracts at a very low figure, but when they 
saw" the market stiffen and their acreage continually reduced, they very 
naturally began to take thought for the morrow. I have conversed 
with hacendados Avho had for some years endeavored to germinate the 
seed, in the hope of solving the problem of cultivating the plant. Lack 
of success, however, was the chief result of siich effort, though a few 
doubtless succeeded in getting plants to grow. Indeed, optimistic state- 
ments as to the possibility of growing the plant profitably have been 
made in some quarters,* and it has even been claimed that the whole 
problem of cultivation at a profit has been solved. As will be seen, how- 
ever, in what fallows, as regards the secretion of rubber, which is the 
all-important point, a very great deal of caution should, in view of the 
lack of evidence, have qualified any statement of this kind. It seems 
more consonant with the truth, as well as with business methods (a 
not invidious juxtaposition, it is hoped), to take a skeptical attitude, 
which, however, need not be unduly pessimistic. It is rash at best to 
attempt to foretell what solution science may bring to any problem. 

ATTEMPTS AT CULTURE. 

That hope has been entertained that the cultivation of guayule on 
a profitable basis may be possible is evident. In addition to private 
owners, at least two companies have spent time and money in seeking 
this end, if unauthoritative statements may be relied upon. Of these 
the Continental-Mexican Rubber Company essayed to make a serious 
trial, and employed a scientific corps to undertake research looking to 
the final solution of the question.^ 

It is not surprising that so valuable a desert plant should have 
attracted the attention of interested persons of other nations whose 
authority extends over desert areas in other parts of the world. No 
detailed statement on this score can be made, however, beyond that the 
Germans ^ are said to be conducting experiments in the cultivation of 
guayule in East Africa. The feeling properly exists that any effort 
toward the subjugation of the desert is justified. The time will come 
when not only those parts of arid regions which may be brought under 
irrigation, but those also which remain unmodified in this regard, will 
yield their possibilities to the hand of man, and Ave stand at this moment 
at the serious beginning of this conquest. 

' See India Rubber World, May i, 1908. 

^ This work has recently been taken up anew (September 19 10). 

^ Ross, 1908. 



CHAPTER II. 
THE ENVIRONMENT. 

GEOGRAPHICAL DISTRIBUTION. 

The northern limit of distribution of the guayule is in the southwest- 
em part of Texas, where it occurs in Presidio, Brewster, and Pecos (near 
Langtry) Counties. This area is continuous with its area of distribu- 
tion in Mexico, throughout which it occurs with greater or less frequency. 
The periphery of this area runs approximately as follows: from the west- 
em extremity of Presidio County in Texas, the western boundary will run 
somewhat west of south till it reaches the northern boundary of Du- 
rango, near Santa Barbara, Chihuahua.^ From this point the limit turns 
approximately toward the southeast, running parallel with the Mexican 
Central Railway at a distance of about loo kilometers (Endlich, 1905). 
Beyond the state of Durango the boundary turns still farther to the east, 
curving northward again not far from the city of San Luis Potosi.^ The 
loist meridian marks roughly the eastern boundary, lying somewhat west 
of it till beyond Saltillo, where the boundary then curves slightly west of 
north, reaching the eastern limit in Texas at about Langtry. The north- 
em limit is marked approximately by Fort Stockton. 

The guayule is thus seen to be peculiar to the Chihuahuan desert. 
The belief which has sometimes been entertained that it occurs in western 
Sonora, southern Arizona, and New Mexico seems not to be well founded, 
and the area within which it is found is confined to the northern portion of 
the central plateau, embracing an area of approximately 130,000 square 
miles, or 290,000 square kilometers. Of this area, it will be understood 
that only a small proportion will be found to carry guayule, and a rough 
estimate of 10 per cent would certainly not be too low. Endlich 's (1905) 
estimate, 75,000 square kilometers, is probably as nearly correct as we 
may make it. It may here be remarked that the very great irregularity 
of distribution makes it very difficult indeed to make anything approach- 
ing an accurate estimate of the amount of guayule as to acreage alone, 
aside from the question of density, so that any figures which may be given 
are subject to correction. 

ALTITUDINAL DISTRIBUTION. 

The whole region in question is, as already said, embraced within 
the northern part of the central plateau (mesa central) of Mexico and 
the adjacent area within which guayule is found in Texas. This area 
has an altitude varying from 2,000 to 10,000 feet above sea-level. The 

' Mr. W. H. Stayton reports seeing a small amount of guayule in the Sierra 
Madre east of Sahuaripa, Sonora. The amount on the eastern slope was somewhat 
greater than on the western. It is now believed to occur sparingly in eastern Sonora. 

^ I am informed that Pringle found guayule near Pachuca, Hidalgo, which is 
probably its southernmost limit. 

13 



1 4 Guayule. 

range of the plant in altitude extends from the lower limit mentioned 
to about 7,000 feet, or somewhat higher. As observed by Endlich (1905)^ 
however, the most important acreage is not usually to be found much 
above 6,000 or 6,500 feet. 

CLIMATE. 

The climatic conditions under which the guayule lives have not 
only scientific interest, but very important practical bearings as well. 
This will be understood upon the reflection that many proposed opera- 
tions relative to the culture of the plant involve the use of water, and 
whatever the theoretical possibilities may be, success on a large scale 
must be conditioned very closely by the nature of the desert areas to 
be utilized. The details in question will be considered in Chapters VIII 
and IX. For these reasons a somewhat detailed account of the actual 
climatic conditions observed at Cedros, in North Zacatecas, will be given. 

RAINFALL. 

Fortunately, perhaps, for our purposes, the year (1907-08) during 
which observations were begun was unusually dry, and afforded, we 
believe, about the most rigorous conditions which the vegetation is 
subjected to without marked unfavorable results. It is to be regretted 
that data for the whole of this year can not be reported, since observa- 
tions could not be commenced before the month of August. Relying 
upon estimates and upon general, verbal reports, and judging by analogy 
with the region about the city of Zacatecas, where the precipitation for 

1907 was about half (320 mm.) of the mean for 29 years (596 mm.),' it 
seems reasonable to believe that the total rainfall for 1907 was not 
greater than 175 mm. (7 inches), of which 138 mm. were recorded in- 
strumentally as falling during the last four months. The growing season, 
as would be indicated by the scant amount of rain which fell earlier in 
the year, was a practical failure as regards crops in general, and the indi- 
cations of growth in the guay;ule, which at this moment concern us most, 
were consonant with the precipitation, which was at best very scanty. 

As will be seen upon examination of table i and fig. 3, the rainfall for 

1908 was somewhat over 10 inches, which appears to be about normal, 
while the effective rains fall in the summer months. In 1908 it was suf- 
ficient to produce a prolonged period of relatively high atmospheric 
humidity, while the replenishment of the store of water in the soil was 
marked enough to produce very pronounced mesophytic conditions. In 
the low-lying flats, especially where the more abundant collections of water 
were formed, annual plants of weedy appearance grew densely breast- 
high, and seedlings of the mariola scattered among them grew with great 
rapidity to a height of 40 to 50 cm. in one season. On the low ridges and 
in the hills the available stratum of the soil was full of water, and the 
guayule and mariola, together with many other shrubs and annuals, 
were in full bloom and making rapid growth in June. Other features 
of the distribution of rains are indicated in a general way in the diagram 
and are of importance as related to the period of growth of the guayule, 
to be referred to beyond. 

^ Boletin Mensual del Obs. Astron. -Meteor. Zacatecas, Jan. 21, 1908. 



The Environment. 15 

Table i. — Rainfall at Cedros, September 1907 to Aiigiist 1908 {fig. 3). 



Date. 


Millimeters. 


Date. 


Millimeters. 

1 


Date. 


Millimeters. 


1907 




1908 




1908 




Sept. 9 


24.4 


Mar. 9 


trace " 


July 7 


9.8] 




Oct. 4 
10 
19 

Nov. 28 


68.3] ' 
1.2 76.7 
7.2 J 

16.2 


^5 
21 

27 

Apr. 3 
9 


trace 

9-4 
trace 


9 
13 
14 
20 
21 


30 
12.8 

24.0 
5-8 
3-4 


^81.6 


Dec. 2 
9 


3-9 " 

7-8 21.3 
9.6J 


II 
17 


7.0 
trace 


22 

27 
28 


trace 
4.8 
9.6 




13 


May I 


3-0 1 


29 


trace 








18 


3 -6 1 „„ ^ 


30 


trace 




1908 




27 


6.0 h9-4 


31 


8.4J 




Jan. 3 
18 


trace 
trace 


30 
June I 


16.8 . 


Aug. I 


4.5I 




21 


trace 


1.2 




6 


3-0 




26 


trace 


4 


2.4 




7 


1-5 


'34-5 






22 


19.2 


43-2 


12 


5-0 




Feb. I 


trace 


23 


14.4 




18 


2.4 




6 


trace 


25 


6.0 . 




20 


17.7 





Note. — It seems probable that the rainfall for the four months of 1907 was 
relatively high, and includes an amount which normally would have been distrib- 
uted earlier in the year, that is, in the summer months. 

I visited Cedros during April 1909. Upon arrival there it was found 
that there had been no rain, save a few drops on a few occasions, be- 
tween August 20, 1908, and April 5, 1909. On the latter date heavy 
showers occurred over considerable areas, leaving water standing in 
"charcos" for several days. This was a very persistent drought, and it 
was found to have affected guayule quite unfavorably in many localities. 
I am informed by Mr. G. R. Fleming that drought again persisted till 
June 16, 1909, when it was broken and a very abundant rainfall ensued 
during the summer of 1909. 

AIR-TEMPERATURES. 

Table 2 shows observed temperatures at Cedros during the time 
indicated. The lacunae observable in May, June, July, and August are 
not as fatal to an adequate notion of the prevailing temperatures as 
might be supposed. A brief study of the table as a whole will show that 
the temperatures are remarkably uniform, and this is especially true of 
the months for which data are lacking. The readings, therefore, which 
were made nearly every day, were not recorded except as they showed 
variations of several degrees. 

The lowest temperatures to which guayule may be subjected are 
not known. The minima at Cedros are undoubtedly higher than those 
which occur in the guayule region of Texas, but as meteorological data 
for that region are lacking we are compelled to judge by those of El 
Paso, the nearest station. The minimum temperatures observed here 
during the last twenty years range close to zero, so that we may infer 
that the guayule plant can withstand lower temperatures than those 



16 



Guayule. 



encountered in North Zacatecas.' Attempts which may be made in the 
future in the cultivation of the plant, ^.g., in New Mexico, must be made 
with regard to its resistance to cold, and it is to be regretted, therefore, 
that a final datum on this point can not be given. 



























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IN. SOIL 






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\ MIN.SC IL- 


^ 


Tj 










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niSht^ 


^7 

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MAY JUNE 



Fig. I. — Maximum and minimum day and night temperatures by months; maximum 
summer and minimum winter soil-temperatures at lo cm. depth. Cedros. 



It will be noted upon examining fig. i that growing temperatures, 
though sometimes low, occur even during the winter months in the day- 
time. At night, however, the air-temperatures are seen to be practically 
non-effective between the middle of September and the beginning of 
May. This condition, judging from air-temperatures alone, may be 
regarded as resulting in a functional resting-period of at least three 
months; that is, the amount of growth possible in the year would be 
that occurring within nine months of time, aside from the considera- 
tion of rainfall. The soil-temperatures are of course higher, and are, on 
account of the high insolation, frequently favorable for the absorption 
of water by the roots, which would, under favorable conditions of soil- 
moisture, be important in respect to the water-content of the plant, 
though it might not, except when water was abundant or under other- 
wise exceptional conditions, stimulate growth. The conditions as re- 
gards growth, then, may be stated thus: The winter, or resting-period, 
is effective during the night-time chiefly during October and on to the 
end of April. The day temperatures during this period may eflfect growth 
when water is sufficient. 



' We now have records showing that guayule can stand a temperature of 5° F. 
at Marathon, Texas, and of 10° F. at Tucson, Arizona. 



The Environment. 



17 



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-t -t LO 'i- m w"/ re "1 ro ^O ^ w"' "■> i/"< Tj- -f 


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cH 


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18 Guayule. 

Table 3. — Maximum and minimum temperatures at Cedros. 



Month. 


Max. 


Min. 


Mean. 


Month. 


Max. 


Min. 


Mean. 


Sept .... 

Oct 

Nov 

Dec 

Jan 

Feb 


"F. 
82 
86 
85 
85 
86.6 

91 


"F. 

64 

53 

39 

26 

22.8 

30 


°F. 

72 

68 

60 

55-9 

57-7 

63.8 


March. . . 

April 

May. . . . 

June 

July.... 
Aug 


"F. 
98 

97 
100 
100 

95 
90 


32 

47 
40 

55 
50 
53 


°F. 
714 
71.9 
74 
75 
72 
67 



SOIL-TEMPERATURES. 

A single record of soil-temperatures extending over a period of 15 
months was made by means of a standard pair of thermometers. The 
instruments were buried at a depth of 10 cm.^ below the surface of the 
ground at station 3. The surface had a gentle slope toward NE. by E., 




Fig. 2. — .A. jr- temperatures it.) and soil-temperatures at 2 cm. depth. November 6, 1907. 

and would therefore receive neither the highest maximum nor the lowest 
minimum insolation. The indices stood at 70° F. when the instruments 
were buried, on December 31, 1907. They were removed April 2, 1909, 
when the following readings were taken: maximum temperature, 94° F.; 
minimum temperature, 52° F. 

' This is about the average depth at which the lateral roots of the guayule 
are placed. 



The Environment. 19 

It is seen, therefore, that at the depth mentioned the lower critical 
growth temperatures in the soil are probably never reached, and it is 
to be inferred that the dormant condition of the vegetation is deter- 
mined by other factors, namely, soil-moisture and air-temperatures, and 
of these the factor of moisture is probably the more effective. 

The temperatures affecting germination, however, are those of the 
surface of the soil or at a very slight depth. Fig. 2 presents the curves of 
air and soil temperatures for November 6, 1907, at a time when difficulty, 
ultimately shown to be due to other causes than temperature, was ex- 
perienced in germinating seeds in boxes. The soil-readings are for a depth 
of 2 cm., and the soil was wet, but was exposed to full insolation. 

The temperatures from about 9 a.m. till 10 p.m. can not be said to 
be unfavorable, though their effect upon the rate of germination and 
subsequent growth would be offset by the succeeding hours of cool soil.^ 
The cooler period is more marked during the succeeding months till 
March or April (fig. i). Inasmuch, however, as the night temperatures 
are scarcely ever favorable for germination (assuming 40° Fahr. as the 
lower limit) before June or after October, and even during this period 
not especially so, we may conclude that the existing temperature condi- 
tions at Cedros are of subsidiary importance in determining the time of 
the year when germination occurs. This conclusion is supported by the 
success attending germination tests made in January (Kirkwood, 19 10), 
when the temperatures ranged from 32° to 64° Fahr. At these tempera- 
tures, germination did not begin so soon as when, later on, they were 
somewhat higher. It therefore may be concluded that, aside from a cer- 
tain rhythm which may be detected, winter dormancy both in the mature 
plant and in the seed is due, in the area we are considering, rather to lack 
of soil-moisture than to unfavorable soil-temperatures. This conclusion 
can not, however, be applied throughout the whole of the guayule region, 
since the winter temperatures in Texas are much more unfavorable. 

SOIL-MOISTURE. 

The residual soil-moisture during sustained periods of drought may 
be reduced to a point below the minimum necessary to sustain life. This 
is the chief cause of the local dying off of guayule during such periods. 
Generally, however, the amount of soil-moisture, though insufficient to 
stimulate growth even if other conditions are favorable, is more than 
enough to sustain life, and indeed may be enough for growth when the 
equilibrium between the plant and the environment is destroyed. The 
results of certain experiments detailed beyond show this to be true. 
Plants at station 2. quadrat 2, were pollarded in November 1907, about 
5 to 8 cm. above the surface of the soil, and these had made a marked 
growth by February 18, 1908, although the surrounding plants showed 
no growth at all, and indeed did not until much later on. While there 
had been a very small amount of rain, it was quite insufficient to account 
for the groAA^h, even in the pollarded plants, during the period between 
the dates above mentioned. We may therefore conclude that usually 



1 Abbe, C, 1905, p. 36. 



20 Guayule. 

during dormant periods the soil-moisture is considerably above the neces- 
sary minimum,' but insufficient to stimulate to growth, although, on 
account of lack of facilities, a quantitative statement can not be made. 
This is to be regretted, because the peculiar distribution of the guayule 
in the foot-slope, while Parthenium incanum extends beyond its limits 
into the play a, ^ is probably connected either with the superior water- 
holding capacity of the soil of the foot-slope or with its greater air- 
content, aside from the differences observable in the topography of the 
root-systems of these plants. The naked statement that the guayule 
is confined to slopes which are well drained^ conveys little of explanation. 

RELATION OF RAINFALL AND TEMPERATURE TO GROWTH. 

Whatever is said here about the behavior of the guayule in regard 
to growth-rhythm must be understood to apply to the region of North 
Zacatecas, where the data which appear beyond in detail were obtained. 
It is believed, however, that the generalizations are approximately true 
for the whole area of distribution.^ 

The grand period of growth falls in the warm season, when super- 
ficial soil-water is normally most abundant and when the night as well 
as the day temperatures are most effective. If the rainfall is subnormal, 
the drought so caused at this time results in very slow growth, made 
possible only by the meager amount of water that reaches the plant 
from the subsoil, derived in part from the earlier and usually small rainfall 
of the previous winter, together with the more immediately available 
supply from insufficient rains. This is only another way of saying that, 
in the region above described, water, as compared with the otherwise 
usually favorable conditions, is the prime condition for gro\\'th, and we 
may best see what the habits of the plant are by observing what growth 
takes place in relation to the rainfall. The extreme possibilities would 
be expected to be shown by plants under irrigation during every season. 
The observed growth in such plants, even in the presence of abundance 
of soil-moisture during November. December, and January, is exceedingly 
small in amount. Had the soil-moisture been reduced, say in Septem- 
ber, so as to bring on a period of dormancy in the plant during October 
and November, it may well be believed that a much more marked growth 
might have occurred during the period following, when in point of fact 
little growth actually occurred. This behavior would be in accord with 
our general knowledge of growth-rhythm. 

Although I have made no observation of positive value in this 
regard, it is said by supposedly competent observers that the guayule 
in the field may be expected to flower at any time, and that it has been 
seen to do so in every month of the year. Flowering, however, usually 
in\'olves some foliage-stem growth as well; and so the evidence favors, 
or at any rate does not contradict, the view that growth may ensue at 
any time of the year. Because of the unfavorable night air-tempera- 

' Cf. Livingston, igo6. 'Escobar, iqio. 

- Tolman, see Spalding, iqog. * Cf. Bray, igo6. 



The Environment. 2i 

lures of the cooler period, however, the total amount of growth will not 
be great, to which result the less effective growing day temperatures 
contribute. The evidence shows further that growth in January, e.g., 
will ensue upon a period of rest coupled with an unusually favorable 
rainfall, spread over time enough to produce a marked rise in the avail- 
able soil-moisture as far down as the shallow roots. The times at which 
this conjunction of conditions may occur is indicated, negatively at 
least, when it is said that no growth in field plants was observed till 
May in spite of the rain, as indicated in table i and the accompanying 
diagram (fig. 3). 

Not only, indeed, did no growth occur, but the guayule plants in 
the field in widely separated localities showed a marked need of water, 
a condition still more evident in April 1909. On the nth of Novem- 
ber, at Jaguey, 10 miles northeast from Cedros, the leaves were in a 
very much shriveled condition. Leaf -fall began toward the middle of 
December, the upper leaves, which are not cast off, being at this time 
in a distinctly flaccid condition. At this time the irrigated plants showed 
signs of leaf -fall, but for some time only the lowermost on the season's 
growth of stem were involved, while in the field plants all the fully de- 
veloped leaves fell away at the same time. 

Although, as above, seen it appears probable that growth may 
take place under favorable moisture conditions even in the winter, 
there is little evidence (Chapter III) that the amount is ever anything 
but small. The internodes are short, and thus is produced a crowding 
of the leaves; which by summer growth would be spread apart, and the 
structural marks between the two grand periods of growth are less ob- 
vious. As will be seen later, the dependence which may be placed in 
these marks as indicating the age of the plant is not materially disturbed 
by this circumstance. 

RELATIVE HUMIDITY. 

Unfortunately no instruments were available at Cedros for the 
study of relative humidity, and it is especially regretted that an atmom- 
eter after Livingston's design was not at hand. The only data obtainable, 
aside from my general observations, are those issued from the Observa- 
torio de la Bufa at Zacatecas. A curve of tentative value based on these 
is presented in fig. 3, and, while this can be regarded as only approxi- 
mate, it serves to indicate that the relative humidity is relatively high 
at Cedros (though not as high as at Zacatecas), and that there is a some- 
what prolonged summer period of high humidity. The following re- 
marks accord in general with these conclusions. 

Dew is frequent during the cooler months, and was sufficient to run 
off the roof of the house occupied as a laboratory, the material being of 
painted canvas. The dew-point is always approached closely at night 
and usually passed in winter and during the rainy summer season. The 
high relative humidities which occur at all times during the night, and 
in certain situations during the day, at least during growing periods, 
are reflected in the vegetation. Only when this factor is taken into 
consideration can we explain the pronounced contrast seen between 



22 



Guayule. 



the vegetations of the north and south facing slopes (Lloyd, 1909), and 
the peculiar distribution of certain plants, notably epiphytic species. 
A most instructive example is offered by Tillandsia ciliata, which is to be 
found epiphytic chiefly on the ocotillo {Fouquieria splendens), on slopes, 
mostly steep, where the drainage of cool air of high relative humidity 
passes downward from higher levels. The ocotillo itself grows in the 
more arid soil of southerly slopes. The Tillandsia {"pastle") occurs on 
other shrubs also wherever the most favorable humidity conditions are 
to be found, namely , in arroyos and narrow canadas receiving air-drainage 
from adjacent high land, and I have seen a small amount in open fiats 
many miles from the mountains, where, during the rainy season, water 
stands for some time over large areas,* thus producing similar conditions 
in less marked degree. 






























3 INCHE 


s 


































Y^'^ 


':-''' 


\ 1 


\ 
















/ 






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-l^ 






\2 INC 


<ES 






/ 


r / 


\ 






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J/' / 








1 


\ ! 






'\ / 


\/-'' 


'^N 








\ 


/ 


1 




1 INCH 


j\ 


/■ \ 


/ 








\ 


/ 


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/ 


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i .1 



JAMi FEB» MAR. APR. MAY JUNE JULY AUG. SEP^ OCT, NOV. DEC 

Fig. 3. — Monthly precipitation at Cedros, and relative humidity at Zacatecas city. 

We may therefore conclude that the atmospheric humidity in this 
region is for a desert markedly favorable for vegetation, and may be 
called into account to explain the denser total growth of this desert as 
compared with the region immediately about Tucson, Arizona. What 
biological relations between plant structure and the conditions described 
above may be found is a problem for the future, the importance of which 
I have elsewhere pointed out (Lloyd, 19086). Ross (1908) refers to the 
occurrence of dews in the guayule region and suggests that the dense tri- 
chome structure may be related to the absorption of atmospheric mois- 
ture, but offers no evidence. At the present time we may do little more 



* As in the "laguna" in the Camacho bolson, east from that place. 



The Environment. 23 

than attribute to the high vapor-tension a general dampering effect upon 
evaporation, both from the plant and from the soil, but it is not improb- 
able that research will discover plant-structures which are specifically 
related to atmospheric humidity, especially as it has been shown (Lloyd, 
1905a) that the ocotillo and probably other plants have the ability to 
take advantage of rain which has not yet reached the earth. 

TOPOGRAPHY AND SOIL. 

The surface of the high plateau of Mexico on which the guayule 
finds its home is broken up into mountain ranges of various extent, sep- 
arated by wide, fiat valleys or "bolsones." The middle reaches (playas) 
of these valleys are nearly level and have a deep, fine, alluvial soil, 
containing a vast amount of capillary water. In this soil the mesquite 
is generally found in abundance, and often of large size. Within these 
flats are frequently found more or less extensive areas (alkali spots, salt 
spots) where salts have accumulated and where the salt-bushes {Atriplex 
sp.) only may be found. 

From the periphery of these alluvial plains, extending to the foot- 
hills of the mountain ridges, is a gentle slope of low gradient, the foot- 
slope, characterized by a gravelly soil (plate 5), which becomes more 
and more stony as the foot-hills are approached. Here the soil is fre- 
quently very shallow and may be confined to the crevices of the under- 
lying rock. This condition becomes still more marked in the hills proper, 
where the edges of the strata are often exposed and where the vegetation is 
confined to the intervening fissures. The most widely distributed plants 
of the foot-slope and adjacent ridges, and therefore the most characteristic, 
are the alvarda or ocotillo {Fouquieria splendens) , the palma samandoca 
{Samuella carnerosa Trelease), and the Cedros sotol {Dasylirion cedro- 
sanum Trelease) .^ The gobemadora or Mexican grease wood {Covillea sp.) 
is also a very common plant of the foot-slopes and ridges, but is to be 
found also in the alluvial plains and is therefore less characteristic. 

Of the species of Parthenium found in the region, the guayule is 
confined to the foot-slopes and foot-hills,^ being also abundant in hills 
not above about 7,000 feet in altitude. It is therefore, like some of its 

* Dasylirion cedrosanum Trelease (n. sp.). 

Subacaulescent. Leaves slightly roughened on the dorsal angles, pale, the 
upper face glaucous, somewhat fibrous-brushy at tip, broad (20 mm.), 1.5 m. or 
more long: prickles mostly 10 to 15 mm. apart, yellow or at length reddened at 
tip, 3 to 5 mm. long, moderately heavy, upcurved or hooked, the whitish-yellow 
intervening margin roughened by minute hyaline tipped denticles. Branches of 
the narrow inflorescence rather elongated, about 7 by 60 mm. Fruit narrowly 
elliptical, 4 to 5 by 7 to 9 mm., deeply and acutely notched, the style much shorter 
than the wings. 

Cedros, Zac, Mexico, Lloyd, No. 118 — the type. No. 82, 1908; Kirkwood, 
No. 96, 1908. 

Allied to D. wheeleri and D. graminifolium, from both of which it differs in 
its smaller fruit not widened upwardly and with shorter style more conspicuously 
surpassed by the wings^(fig. 4, and on the extreme right of fig. A., pi. i). 

The type is in the Herbarium of the Missouri Botanical Garden; cotypes in 
the Gray and National herbaria. 

* It is generally believed by those familiar with the plant that it affects more 
particularly the south slopes, and this accords in general with my observations, 
though it must not be inferred that it does not grow at all on north slopes. 



24 



Guayule. 



associates above mentioned, an "edaphic" species, found only where 
the ground is stony. In the alluvial plains one meets only an occasional 
isolated plant, but if the plain is traversed by a low ridge of gravelly 
ground, even if the surface is raised only a few inches above the surround- 
ing area, the guayule may be found. In the fine soil of the plain, on the 
other hand, the mariola {Prathenium hicanum H. B. K.) and the annual 
species P. hysterophorus grow in abundance, though the mariola is com- 
rrionly associated with guayule on the foot-slopes and hills. This asso- 
ciation of guayule and mariola frequently misleads the inexperienced 
observer in estimating the amount of guayule which may be found in a 
given area. 

Why the guayule does not grow in the fine alluvium is not clear, 
and is a question often asked by persons familiar with the facts. Any 
reasons, aside from those mentioned above, which may be assigned are 
at present of only speculative value, but some reference may properly 
be made to them. 






Fig. 



-Dasylirion cedrosanutn Trelease. Type material in the lower row. Above, for comi>arison 
fruits of D. wheeleri at the left and of D. graminifolium at the right. X 3/1. 



Guayule is confined practically to the Cretaceous region of the Cen- 
tral Plateau, and therefore to highly calcareous soil (see Chapter IX). 
It may very well be that the plant is sensitive to even a slight acidity, 
and therefore prefers a soil with a very small amount of humus. Certain 
experimental results referred to beyond, while not conclusive, indicate that 
this explanation may apply during the period of germination, but it has 
been found that the absence of lime is not a hindrance to maturer plants. 

It is a popular notion that the plant "rots" in situations where 
water is relatively abundant, and that for this reason it is not to be found 
in "bajillos" or low places. It is true that for a considerable period in 
the summer season practically mesophytic conditions prevail in many 
areas within the fiats, especially in the frequent slight depressions. Here 
annual weeds groAv in profusion, and a number of species of desert shrubs 
flourish. Among these is the mariola, the seeds of w^hich germinate 
freely among the dense vegetation of shrub and weed, and in one season 



LLOYC 



PLATE 5 



^S!T 



W^fWeBSOK- 





K^i% J* "•■ n4li«iao>,i' V -Vol's ^"^ 'Vj'W'v > ^ -;.* '^™?-'I^-CH»\ St^ 

w^i-«- - W^ 'I'-i^ : #^^:#kYa V'^^-'^'.y ^^iwl^^V^d 




A. Station 2, Quadrats 5 and 6, foot-slope of Sierra Zuluaga. 

B. Station 3, Quadrat 1, near Cedros. A good stand of mature plants. 




Plants from Quadrats 5 and 6, Station 2. 



The Environment. 25 

attain a height of a foot or two ; but this is not true of the guayule. That, 
however, the mere quantity of water or the density of the vegetation 
are not the determining factors is shown by experimental evidence, 
while in the field are to be found numerous instances of plants which 
have germinated in the dense shade and dampness found beneath the 
dead leaves of the sotol and in crowded conditions produced by other 
plants, such as the lechuguilla. Indeed, these are frequently the only 
conditions under which the plant gains a foothold. It therefore does 
not appear probable that the abundance of water or the density of the 
vegetation is the determining factor in preventing the guayule from get- 
ting a start; hence we may infer that the conditions below the surface 
must be understood before an explanation may be had. The edaphic 
habitat of the plant suggests that the mechanical conditions of the allu- 
vial soil are unfavorable, owing to meager aeration, in connection with 
which the humus conditions also may have to be taken into account. 

DENSITY OF GROWTH. 

Of great importance economically as w^ell as to the student of vege- 
tational problems is the number of plants per unit of area, both abso- 
lute and relative. The operations of the forester rest upon this datum 
in the first instance, as this, together with the size of the individuals, 
forms the basis of calculations of the available tonnage per acre. It 
will readily be understood that any estimate on a large scale will involve 
a necessarily large error, since it would be impossible to do more than 
proceed on the basis of sample counts combined M-ith acreage and esti- 
mates of size. This can frequently be done with great accuracy by persons 
who have had practical experience in taking guayule from the field, 
especially if the judgment be checked by survey and sample counting 
and weighing. The following tables, the data for which were obtained 
by accurate measurement, will, however, serve a useful purpose in indi- 
cating a method of making estimates, as well as in furnishing indications 
of actual conditions. For the purpose, quadrats of loo square meters 
were laid out by means of a steel tape, the data obtained attaching to 
the guayule plants within each such quadrat. 

The weights in the following tables are field weights. For dry 
weights a reduction of 20 to 25 per cent is necessary. As field weight is 
usually assumed, however, I have followed the usage and have not applied 
the above correction. 



26 



Guayule. 



Table 4. 
Two adjoining quadrats, each of 100 sq. meters, on a loma or ridge extending 
toward the Sierra Zuluago, about 10 miles north of Cedros (plate 5, fig. A). All the 
plants were pulled up and sorted, each package containing plants of similar size and 
habit. The packages were then grouped into classes arbitrarily, and a typical plant 
for each class photographed (plate 6) . The age of this plant was carefully estimated 
and checked by estimating the ages of a number of similar plants. (March 29, 1908.) 



Class, 


No. of 
plants in 
package. 


Weight of 
package. 


Average 

weight of 

individuals 


Estimated 
age. 


Average 

height of 

class. 


I 

II 


• 


20 
20 
20 
20 

20 
20 
20 
20 
20 
20 

50 
40 


lbs. 
IS 

17 
22.5 

17 

13 
10.5 

9 
10.5 

9.5 
12 

19 
10.25 

II. S 
10 

8.25 
18 

7-5 
II 

7 
8 
6 

7.75 
18.25 

12.5 
10 
2 


lbs. 

0.7S1 
0.85 , 
1 . 12 
0.85 . 

0.65 ' 
0.52 

0.45 , 
0.52 
0.47 
0.6 

0.38' 
0.25 , 
0.29 
0.25 . 

0. 14 " 

°-^s . 

0. 14 
. t6 

0. II ' 

0. 1 

0. 1 

0.08 

> 
0. 1 

. 12 

. 1 

oil. 


years. 

15 to 18 

II to 13 

7 to 10 
5 to 7 

I to 5 


cm. 

58 

30 

25 
20 

15 


Ill 


IV 


1 40 
I 40 

r 60 
120 


V 


• 


50 

I 70 

r 60 

80 

60 

100 

183 

100 

100 

18 


Total 1 


1371 


303.0 

















1 68s plants in loo square meters. 



Class V is made up of all sizes up to the maximum indicated. In plate 
6, figs. 5 to 7, examples of three sizes and ages are shown. The weights 
and estimated ages of all the plants in plate 6 are shown in table 5. 





Table 


S- 




Class. 


Age. 


Height. 


Weight. 


I 


years. 

15 

II 

8 

7 
S 
2 

I 


cm. 

S8 
30 

25 

20 

IS 


oz. 

18. 5 
6. 

1.87s 
0.875 
o.S 
0.05 


II 

Ill 

IV 

V 


VI 

VII 



The Environment. 27 

Table 6. — Station 8, quadrat 2 (100 square meters on low ridge just north of Cedros). 



Class. 


No. of 

plants in 

class. 


Total weight. 


Individual 
weight. 


Height. 


Age. 




/ 


b5. 03. 


lbs. oz. 


cm. 


years. 






I 


3 14 


3 14 


70 


(0 


I 




I 
I 


3 10 
3 10 


3 10 
3 10 


60 
70 


20 
22 










I 


3 6 


3 6 


60 


15 to 18 






3 I 


8 


3 8 


60 to 68 


20 






4 


9 14 


2 7-5 


45 to 55 


16 to 18 








2 4 


2 4 


68 


20 (about) 


II 




2 4 


2 4 


55 


20 




2 8 


2 8 


50 


IS to 18 






2 10 


2 10 


50 


IS to 18 






2 10 


2 TO 


70 


(0 






. 4 


8 14 


2 3-5 


45 to 50 


' 18 to 20 






4 


6 8 


I 10 


40 to 45 


15 






I 


I 4 


I 4 


46 


15 






I 


I 14 


I 14 


65 


1 7 to 1 8 


Ill 




4 

I 


6 14 
I 00 


I II. 5 
I 00 


45 to 50 
40 


15 
7 or 8 








I 


I 3 


T- 3 


37 


7 or 9 






I 


I 6 


I 6 


40 


10 to 12 






I 


I 7 


I 7 


43 


12 






I 


I 00 


I 00 


35 


9 






2 


I 14 


•• 15 


40 


7 to 8 






2 


I 14 


•• IS 


35 


* 10 to 15 


IV 




I 
I 


II 
1 1 


II 
II 


35 
40 


10 
8 








2 


I 6 


II 


38 


'15 






2 


I 4 


. . 10 


40 










8 


8 


35 


'9 to 10 








6 


6 


30 


9 








4 


4 


30 


8 








3 


3 


30 


10 








2.7s 


2.375 


27 


6 to 7 








2.125 


2.125 


23 


5 


V I 




2.125 


2.125 


27 


6 to 7 






1-5 


1.5 


20 


Q) 


j 


2 


3-75 


.. 1.87s 


27 


6 to 7 






I 


1-125 


1. 125 


20 


5 






2 


•875 


I. 5 


13 


3 to 4 






. 14 


{') 






I 




3* 1 


(') 








Total 


75 ^ 


53 14 







1 Old scraggly plants whose age it was impossible to determine even approximately. 

2 Slowly growing, densely branched plants. 
' Tall and slender. 

* Retonos. 

s Very small. 

•• Classes I-IV, inclusive. 



28 



Guayiile. 



Table 7. — Station S, quadrat i (100 square meters on low ridge just north of Cedros, 

July 20, 1908). 



Class. 



II 



III 



IV. 



V 



Total 



No. of 
plants in 
package. 



10 
10 
10 



Total weight. 



lbs. 
6 
6 
4 
4 
4 
4 
4 

3 
3 
3 
3 
3 



10 
10 



10 
10 



5 
5 

81 135 
1 Retofics. 



oz. 
4 
4 



14 
10 
10 
4 
00 

14 



00 
00 
00 



6 
10 



00 
8 

00 
4 



Individual 
weight. 



lbs. 
6 
6 
4 
4 
4 
4 
4 

3 
3 
3 
3 
3 



OS. 

4 
4 



14 
10 
10 
4 
00 

12 



4 
00 
00 
00 



3 

6 

10 

12 

13 



3 
0.8 



Height. 



cm. 
70 



60 to 70 
60 to 70 

60 

40 to 45 
50 to 60 

55 



45 
45 



45 to 60 

35 
30 to 35 

30 
15 to 20 



The Environment. 



29 



Table 8. — Station 9, quadrat i (100 square meters, on a 2^-degree northeast slope, 
in the hills east of Cedros) . 



Class. 



II 



III 



IV. 



Total 



No. of 
plants in ! Total weight. 
package. 1 



20 
20 
20 

4 
8 

I '4 



lbs. 

3 
3 

4 
4 
4 



63 



10 
2 



325 



1-25 



•375 
• 31 
•125: 

.I251 



Individual 
weight. 



lbs. 
3 



14 

13 

10 

8 

5. 
4 



15 
10 

8, 

5 

3 



5 

2.125 
r.625 
1-5 
1-375 

1-25 

1. 18 
.875 
■75 
.625 

• 5 
•25 
.06 

• 03 

• 015 



' Retofios of small size. 

2 Total weight, classes I to III, inclusive. 



Height. 



cm. 

60 to 65 

60 

50 to 60 

55 

3 5 to 40 

60 

45 

40 to 45 
50 
50 
60 
40 
45 
45 
40 

35 

45 

25 to 3 5 



30 



Guayule. 



Table 9. — Station 2, quadrat 7, April 3, 1909. 

[The data for this table were obtained by pulling up, sorting, and weighing all the 
plants on 100 square meters in a guayule field from which all the plants above 
40 cm. tall had already been taken. (Plate i, fig. A.)] 



No. of plants. 


Weight. 


Average weight. 


Height. 




lbs. oz. 


oz. 


cm. 


100 


25 8 


4 


25t035 


60 


II 


3 


30 


60 


12 


3-2 


25 to 53 


60 


10 8 


2.8 


20 to 35 


31 


7 4 


3-7 


25 to 30 


60 


13 • ■ 


3-5 


22 to 30 


50 


4 8 


1-4 


20 to 30 


50 


7 4 


2.32 


20 to 25 


50 


6 12 


2 


20 to 25 


50 


4 4 


I . 26 


20 to 25 


50 


5 8 


1.76 


20 to 25 


64 


3 4 


0.8 


18 to 20 


61 


2 


0-5 


15 


^9 


0.125 






755 


112 12.125 







1 All seedlings of 1908, except one of 1907. 



Table 10. — Station 9, quadrat 2, April 14, 1909 (100 square meters, ridge of loma in 
hills (El Potrero) , east of Cedros) . 



No. of plants. 


Weight. 


Average weight. 


Height. 


Remarks. 




lbs. oz. 


oz. 


cm. 






4 4 


68 


70 






3 


48 


65 






3 


48 


65 






2 8 


40 


63 


Scrubby. 




3 12 


60 


62 






I 12 


28 


60 






2 4 


36 


60 






2 


32 


57 


Rather scrubby. 




I 12 


28 


55 


Do. 




2 


36 


53 






5 12 


92 


50 


Spread 100 cm. 




2 4 


36 


50 






2 12 


22 


44 




4 


3 12 


15 


40 






3-5 


3-5 


24 


4 to 5 years seedling. 




I 


T 


24 


Badly developed. 




2.5 


2-5 


23 


Seedling, 5 years. 




2.5 


2-5 


16 


3 years retoiio. 




0.5 


05 


1 1 


Seedling, 2 years. 




0.5 


0-5 


14 


Seedling, 3 years. 


24 


41 6 







The Environment. 



31 



Table ii. — Station lo, April 5, 1909 {quadrat of 100 square meters). 
[On a southerly slope 10 kilometers north of the Cerritos de los Calzones.] 



No. of plants. 



2 

7 
5 
7 
10 
8 

5 
16 

5 
10 
10 
16 
12 
6 
7 



186 



Weight. 



lbs. 

2 

4 
6 

9 
6 

5 
6 
6 

6 

5 
5 
6 
6 
3 
4 
4 
3 
2 
2 
'4 



Average weight. 



oz. 

36 

8 

23 
19 
29 



12.5 



Height. 



4 




14.8 


18 


12.6 


9.6 


13 


II . 2 


4 


13 


4.8 


4 


4 


2 


75 


5 


3 





6 



65 

50 

40 to 50 
40 to 50 

45 
35 to 50 
30 to 50 
35 to 40 
3 5 to 40 
30 to 40 
30 to 40 

35 

35 

35 

35 
30 to 3 5 
3ot035 
25t035 
20 to 40 
25 to 30 

25 

20 to 30 
15 to 20 



Remarks. 



Note. — The shrub of 
this region is rough 
looking and rather badly 
attacked by insects. A 
good deal of witches' 
broom, and many plants 
attacked at the base by 
borers. 



Seedlings, 2 to 3 years old. 
Seedlings, 2 years old. 
Seedlings, about 3 yrs. old 
Retonos. 
Small plants. 



1 Scant. 
Table 12. — Station 11, near Caopas, April 6, 1909. 



No. of plants. 


Weight. 


Average weight. 


Height. 




lbs. oz. 


oz. 


cm. 


I 


2 6 


38 


50 


I 


I 10 


26 


50 


5 


5 8 


17.6 


40 to 50 


5 


7 4 


22 


35 to 40 


6 


6 


16 


35 to 40 


I 


I .... 


16 


35 


8 


6 


12 


30 to 40 


5 


5 


16 


30 to 40 


7 


5 • ■ • • 


II. 4 


30 to 40 


8 


4 


8 


30 to 35 


9 


2 8 


4-4 


3ot035 


« 


2 8 


5 


30 


12 


4 12 


6.3 


30 


15 


2 .... 


2 . 1 


25 


21 


3 •••• 


2.3 


20 to 30 


12 


2 8 


3-3 


20 to 25 


37 


3 8 


1-5 


20 to 25 


10 


2 8 


4 


20 to 25 


25 


2 4 


1-4 


20 


30 




0.5 


20 


17 


10 


0.6 


15 to 20 


21 


4 


0.2 


15 


3 


2.5 


0.8 


15 


12 


. . 10 


0.8 


10 to 15 

_ — ^1 


279 


'71 14-5 





1 7,100 pounds per hectare. 



32 



Guayule. 



Table 13. — Station 12, foot-slope ridges south froin Apizolaya, 
April 10, 1909. {Plate 7, and plate i,fig. B.) 



No. of plants. 


Weight. 


Average weight. 


1 
Height. 




lbs. oz. 


oz. 


cm. 


2 


4 8 


35 


60 to 70 


5 


10 .... 


32 


60 to 70 


5 


10 


32 


60 to 70 


5 


Q 8 


30-4 


60 to 70 


5 


9 8 


30-4 


60 to 70 


5 


9 


28.8 


60 to 70 


5 


9 


28.8 


60 to 70 


5 


8 


25.6 


60 to 70 


5 


7 12 


24.8 


60 to 70 


5 


7 


22 .4 


60 to 70 


• 5 


6 8 


20.8 


60 to 70 


I 


3 


48 


60 


5 


9 8 


30-4 


60 


9 


6 


10.7 


60 


5 


2 8 


8 


50 


7 


3 


7 


50 


9 


4 


7-r 


50 


7 


3 8 


8 


50 


5 


4 4 


13.6 


40 to 50 


6 


4 S 


'^ „ 


■ 45 


5 


4 


12.8 


40 


5 


3 8 


II .2 


40, 


5 


4 4 


13.6 


40 (i at 50) 


10 


3 


4.8 


40 


6 


5 4 


14 


35 to 45 


10 


2 4 


3.6 


35 to 40 i 


12 


2 4 


3 


30 


25 


2 


I -3 


30 


20 


3 4 


2.6 


30 


25 


2 8 


1.6 


30 


28 


I 12 


I 


2 5 to 3 


20 


2 12 


2 . 2 


2 5 to 3 




. . 10 


5 


30^ 


13 


6.5 




18 to 23 


4 


2.75 




18 to 23 


^ , 5 


• • . . 3-5 




22 


Add, makin^ 


2[ additional we 


ight— 




10 


3 4 




40 to 60 (scrappy) 


Scraps . . . 
311 


4 






172 5 





Summarizing the above results, and including data from other sources, 
we have as follows: 



Station. 


Quadrat. 


N'o. of plants. 


Station. 


Quadrat. 


No. of plants. 


4 


I 


50 


8 


I 


81 


5 


I 


275 


Q ! 


I 


130 


2 


3 


360 


2 


7 


755 


2 


■ 4 


270 





2 


■ 24 


2 


I 


285 


10 




186 


3 


I 


30 


1 1 




279 


2 


^ 5 and 6 


68s 


12 




Z''^ 


8 


2 


75 


, i 







'Averaged. 



LLOYD 




vri|^^ 



^ 









^^-./ 






IT', 



..■v'V. 



*«.«. .#,/• ,-#3^::. iif 













~V.4*t.>«^'Wi 



A. Quadrats (station 12) in a very dense growth. Apizolaya. 

B. The same, the guayule removed. 



The Environment. 



33 



Here there is a range in numbers of plants from 2,400 to 75,500 
plants per hectare, but the meaning of these figures can not be under- 
stood unless the size of the plants is taken into consideration. From 
the point of view of business opportunism, a stand of 2,400 plants per 
hectare may be better than one of much higher figures, while for one who 
is looking for a basis for permanent investment other questions of rela- 
tive sizes and numbers of plants arise, the answer to which involves an 
explanation of the rate of reproduction in the field. This subject will 
be treated in detail in Chapter IV, it being our purpose here to show 
the actual condition as viewed by one who is estimating the tonnage per 
unit of area. 

If we refer back to table 4, we will observe that the two quadrats 
contained 1,371 plants, the average weight of which was a little over 3.5 
ounces. Of these, however, only 80 were large enough to be gathered, 
namely, those about i pound or over in weight; though if the land were 
being exploited smaller ones would be taken, say those weighing above 
half a pound. This would include all of the plants in classes i and 1 1 , 
weighing in the aggregate about 58.75 pounds, or 5,875 pounds per hec- 
tare, assuming the quadrats to be fair samples, or about 2.67 tons (long). 

Treating the remaining tables similarly, we have the following 
figures : 

Table 14. 





No. plants 


Weight of 
these. 






No. plants 


Average weight 


Table No. 


above 


Weight per hectare. 


below 


of available 




8 ounces. 






8 ounces. 


plants. 






lbs. oz. 


Ihs. 


lens {long). 




lbs. oz. 


4 


80 


S8 12 


5.375 


2.4 + 


605 


11.75 


6 


45 


83 14 


8,390 


3-7 


30 


I 13 


8 


7 


69 


131 7 


13.140 


6 - 


12 


I 14 


5 


8 


43 


6q 8 


6.950 


3-1 


90 


I 9 


8 


10 


18 


40 5 


4,033 


1.8 


6 


2 5 


8 


II 


114 


Si 8 


8,350 


3-7 


98 


II 


7 


12 


47 


43 12 


4.374 


2 — 


232 


14 


9 


13 


113 


137 


13.700 


6.1 


200 


I 3-4 



It will need but a glance at the above summary to show that, from 
the business point of view, the acreage of large but comparatively few 
plants is the more valuable to the purchaser who is not looking to the 
future, for the reason that the cost of harvesting a small number of 
large plants will be less than if the available plants are large in number 
and of smaller size, and because the larger plants can be handled more 
readily and therefore more cheaply. Furthermore, it is much easier 
to determine the tonnage with fair accuracy where the plants are few 
and large. The error due to applying data taken from small sample 
areas to an extensive area within which the sample area falls, must of 
necessity be large, for the number of plants as well as their character 
must be considered. Taking the question of number alone, the size of 
the error on this score will be appreciated when it is known that on an 
area of 42.7 acres at Station 2 (plate i) 181 bales of guayule, or at the 
3 



34 



Guayule. 



rate of about 800 pounds per acre (i ,976 pounds per hectare) , were actually 
collected. As this was gathered under the rule that no plants less than 
40 cm. in height or in spread were to be taken, some plants which would 
run over 8 ounces were doubtless left, but allowing for this error probably 
not more than 2,000 pounds to the hectare could have been taken, or at 
most I ton of 2,200 pounds. On another area of 30.8 acres of the same 
general character, but of thinner stand, 53 bales or at the rate of 344 
pounds per acre (about 850 pounds per hectare) were gathered. 

It will thus be seen that the difficulty in estimating tonnage per 
unit of area with small error is at best very great, and this, as already 
said, is rendered more so by the difference in the character of the plants. 
To judge of the truth of this, one has but to examine the various illus- 
trations accompanying this paper. In particular, a comparison of two 
prevalent types is shown in plate 8, namely, a slender and a spreading 
type, but neither of extreme form. 

Table 15. — Dimensions of narrow and spreading types of shrub, illustrated in plate 8. 



Narrow type. 


Spreading type. 


Plant. 


Weight fresh. 


Weight dry. 


Height. 


Plant. 


Weight fresh. 


Weight dry. 


Height. 




lbs. oz. 


lbs. oz. 


cm. 




lbs. oz. 


lbs. oz. 


cm. 


A 


4 


2 II 


65 


A 


3 6 


2 4 


50 


B 


2 


I 2 


48 


; B 


2 12 


I 13 


45 


C 


I 2 


■ • 15 


46 


! c 


I 


. . 14 


33 


D 


. . 8 


5-5 


33 


D 


6 


5 


23 


E 


.. 6 


4 


28 


E 


5 


3-75 


21 


F 


- • 3 


1.5 


24 


F 


1-75 


0-93 


17 


G 


.. 1.25 


0.5 


23 











From the above data it is seen that, speaking broadly, the weight 
of plants of the spreading habit is one-third to one-half greater than those 
of the narrow type of similar height, so that a stand of the latter must 
have a density correspondingly greater to equal in total weight a given 
stand of the spreading type. 

As one looks over a "field" of guayule, these apparently minor dif- 
ferences of form are not at all apparent, because of the interference of 
other vegetation with the vision. If the occasion presents itself when 
more accurate estimates will be demanded than at present, this condi- 
tion will have to be taken into account. It should be further mentioned 
that the weights given above are of freshly gathered plants. If it is 
desired to calculate to "air-dry" shrub, the proper correction should 
be applied, but as this is very variable, according to the season, no con- 
stant can be given. It may, however, be as great as 22 per cent in the 
dry season. 

The only other pubHshed calculations of this kind were made by 
Endlich (1905, p. 11 18), who, for the purpose of calculating the area of 
guayule land necessary to support the industry, assumes the average 
weight of the plant to be 500 grams, and the density of growth to be, 
by weight, 500 to 800 kilograms per hectare, or from 1,000 to 1,600 



The Environment. 



35 



plants per hectare of 500 grams average weight, taking into account the 
unevenness of distribution, that is, the more or less extended areas where 
guayule does not occur. The following figures are deduced from the 
quadrats above detailed, taking all the plants into account : 

Table 16. — Number of plants in given areas. 



Table Nos. 


No. of 


Average 


weight. 


Kilograms 


recording 


plants per 






per 






quadrats. 


hectare. 


Ounces. 


Grams. 


hectare. 


4 


68,500 


3-5 


99.2 


6.795-2 


6 


7.500 


18.4 


521.6 


3.912 


7 


8,100 


26.56 


7530 


6,099 


8 


13.300 


8.5 


241 .0 


3.205 


10 


2,400 


27 . 2 


771. I 


1,850.64 


II 


18,600 


8.71 


246.9 


4,592.34 


12 


27,900 


4.12 


116.79 


3.258-72 


13 
Ave. . 


31,100 


8.86 


252.97 


7,867.36 


22,175 


13.2 


375-3 


4,672.53 



From the above it is seen that the average in long tons per hectare 
is 4.67, per acre 1.85. 

The average weight of all the plants on the quadrats is thus seen 
to be less than Endlich's estimate by 125 grams, or one-fourth, and as 
these sample areas include the very best guayule land, that is, the densest 
areas with the largest plants in relation to the density, it may be con- 
cluded that the present estimate is more nearly correct. In estimating 
the average density over large areas, great difficulties are met. Endlich 
assumed one-tenth of the area of the guayule region to be occupied by 
the shrub at an average density of 500 to 800 kilograms per hectare. 
This figure does not approach the indications of our data, though it 
must be remembered that these do not take into account poor areas 
where the shrub is very scattering or nearly absent — as the Mexican 
well expresses it "salteadito." For certain areas, e.g., one of 1,800,000 
acres (728,744 hectares) which has been somewhat closely studied for 
the special purpose of estimating the amount of shrub to be found there, 
Endlich's factor was found to be very small, for if only one-hundredth 
of its area carried guayule in the quantity of our general average, there 
would be as much as of one-tenth of it which carried shrub of the amount 
of his factor. We may feel sure, however, that our average applies to 
more than one-hundredth of the total area. Whether Endlich's figure 
applies better to the total guayule area of Mexico can not be said with 
any certainty, but it is only fair to say that, in view of the great diffi- 
culties involved, it is probably as near the truth as any that we might 
venture. 



36 Guayule. 

BIOTIC RELATIONS. 

COMPETITION. 

The relation of guayule to the other plants with which it is com- 
monly found associated is of great importance, especially if forestry 
methods are contemplated. Both the mutual effect of each element in 
the vegetation upon the guayule and the relative rate of growth must 
be understood in order to judge what the final effect in the struggle for 
existence is likely to be. To do this, however, involves a very consider- 
able amount of sustained observation by means of the quadrat method, 
first devised by Clements. Following is a census of the more important 
plants found growing in association with the guayule in quadrats 5 and 

6, Station 2. 

Table 17. 



Scientific name. 


Common name. 


Total No. in 200 
square meters. 


Parthenium arpentatum . . 


Guayule 


1371 

50 

6 

4 

I 

Scattered all 
over. 

6 

about 20 

45 




Lechuguilla 


Covillea mexicana. .......... . . . . . 

Samuella carnerosa 


Gobernadora. . . . . 


Palma samandoca 

Sotol 






Huisache 




Sangre de drago 








Pevote (pevotl) 




Rastrero 







Also the following, from Station lo: 

Table 18. 



Scientific name. 


Common name. 


No. in 100 

square meters. 


1 


186 
14 

5 
7 
8 

I 

3 

I 
Several small 
inconspicu- 
ous plants. 




Mariola 




Nopal Colorado 




Segador 




Gobernadora 


Opuntia inibricata 


Gardenche ■ ■ 


Engorda cabra 




Sotol 


Samuella ca,rnerosa 


Cacti . 


Palma samandoca. 
Maguey. 


Ao'ave asperrima 


1- 



With few exceptions, these constitute the dominant vegetation of 
the foot-slopes and the low ridges, though of course a number of other 
species may be found in other locahties, and indeed may be more impor- 
tant elements elsewhere than has been observed to be the case in North 
Zacatecas. 



LLOYD 





A. Narrow type of guayule. B. Spreading type of guayule. (See table 15.) 



The Environment. 37 

A few of the more obvious of these are: 



Scientific name. 


Common name. 


Kchinocactus palmeri 


Biznaga burra. 
Ocotillo or alvarda. 


Fouquieria splendens 


Echinocactus pringlei 

Buddleia marrubiifolia 


Biznaga colorada. 
Asafran. 



The above enumeration indicates that at the present time the guay- 
ule in this habitat is far and away the most important plant numerically, 
and is therefore dominant in the usual sense. Whether it will continue 
so — whether its dominance is waxing or waning — may be indicated by 
the relative numbers of guayule plants of different ages and by the inter- 
action of the various elements in the vegetation. 

We may therefore consider briefly each of the numerically most im- 
portant species. 

Lechuguilla (Agave lecheguilla). 

While the actual number of plants of this species found in quadrats 
5 and 6 is much larger than that of any other save guayule, it is very 
small compared with the number which is found on much guayule land 
{e.g., plate 5, fig. B). 

In common with the Agaveae, the plant propagates itself chiefly by 
means of stolons which lie a few centimeters below the surface. In this 
way it spreads from an original plant radially, taking up the ground as 
it goes, from which nothing but death dislodges it. In the course of a 
few years it attains maturity, when a tall flower-stalk is developed; then 
the whole individual, consisting of a single cluster of leaves attached 
to a short (10 to 15 cm.) and thick (6 to 7 cm.) stem, dies. Where the 
lechuguilla has occupied the ground for some time, it frequently forms 
a dense growth, from which other plants, save a few annuals or emaciated 
perennials, are excluded. Its manner of spreading, by which it repro- 
duces itself vegetatively, enables the plant to occupy areas in which the 
soil is confined to the crevices of the rocks, and in this manner it may 
occupy ground which is unfit even for those desert plants with which 
it is usually associated. From it is extracted the fiber "ixtle tula," or 
"ixtle de lechuguilla," which is of considerable commercial importance, 
and thus the plant is of some value — not, however, sufficient to justify 
it as a competitor of the guayule. The method of vegetative reproduc- 
tion above noted is also characteristic of the guayule (Lloyd, 1908c), 
especially when growing where the country rocks come to the surface, 
but is in this plant of relatively much less importance. 

The mutual behavior of these two plants under strong competition 
is not very easy to describe precisely. It seems clear that, with the excep- 
tion of a few plants which succeed in gaining a foothold by germinating 
in the shade between plants of lechuguilla, sometimes being favored 
by the protection from drying out and from cropping by animals thus 
afforded, ground occupied by lechuguilla is much less favorable for the 



38 Guayule. 

growth of guayule than that from which lechuguilla is absent. For 
although it would seem that germination and early growth are favored 
by the protection offered by the lechuguilla, as a matter of observation 
one finds but few young plants of guayule in such situations. One reason 
for this is, probably, that the guayule seeds (achenes) find difficulty in 
reaching the soil, because the leaves of the lechuguilla catch them and 
hold them in their axils till they die, thus materially reducing the num- 
bers which reach the ground. Aside from the consideration that the 
lechuguilla takes up from the soil its quantum of water, its effect upon 
guayule is unfavorable, therefore, because of its superior powers of pro- 
gressively and steadily occupying the ground, and because of the loss 
of guayule seed by being caught in its leaves. Lechuguilla appears to 
be an increasingly dominating type in every situation where it gains 
a foothold. It is common to every part of the foot-slope and in the 
hills throughout the range of guayule. The great quantity of it to be 
found produces in many parts of the mesa central the dominating yellow- 
green coloring often seen there. When it and the guayule are associated, 
the green is dotted by the gray of the latter, although other plants also 
may contribute this subdued note in the coloring. 

GOBERNADORA (COVILLEA TRIDENTATa) AND OCOTILLO (FOUQUIERIA SPLENDENS). 

These may be considered together. Their forms are similar because 
of the habit of their slender branches, which arise from near the base 
and reach obliquely upward, producing the effect of an inverted cone. 
They are both taller than guayule, but the shade cast by them is small 
in amount, and less is cast by the ocotillo than by the gobemadora. The 
only places where the ocotillo grows thickly are in certain situations on 
south slopes, and here it often forms a dense thicket. When thickly grow- 
ing it would interfere with the rapid harvesting of guayule because of the 
thorny branches, but, excepting for the draft it makes on the soil for 
water, the effect upon guayule is negligible. This applies about equally 
to gobemadora, which in North Zacatecas, however, reproduces itself quite 
rapidly by seed, and so may readily come to occupy too much ground. 

Palma samandoca (Samuella carnerosa) and Sotol (Dasylirion cedrosanum). 

These are similar in form. Each plant has a single stem supporting a 
large rosette of leaves. The sotol, however, rarely rises sufficiently above 
the surface of the soil to free the surface from the lower dead leaves, 
which cover about lo square feet of area. Both plants are valuable eco- 
nomically, the palma samandoca affording a fiber of less value than the 
lechuguilla, but of which a good deal is prepared, while the other is the 
basis for the manufacture of the whisky-like liquor, mescal sotol, or simply 
sotol. Neither of these occurs in sufficient numbers to figure in compe- 
tition with the guayule within its proper habitat. Indeed, for reasons not 
yet understood, when sotol grows densely, forming a chaparral, guayule is 
entirely absent. One reason, if not the only important one, is that the 
sotol appears not to be confined to limestone areas, but is not excluded 
from them. 



The Environment. 39 

Sangre de drago (Jatropha spatulata). 

This plant is a very characteristic xerophyte, and is found beyond 
the Hmits of the Chihuahuan desert, westward into Sonora (MacDougal, 
1908). The upper part of the plant consists of a simple, dark-brown 
and somewhat fleshy stem, scarcely branched at all and slightly curved. 
The leaf-producing lateral shoots are very short, and are roughened with 
small scales; from them arise the bright green narrow leaves in clusters. 
Reproduction takes place readily by means of seed, and the plant spreads 
by underground stems which are thick and fleshy, and are, in fact, water- 
storage organs. Like the lechuguilla it is a colonial form, growing in 
dense patches, but is less able to occupy the ground to the exclusion of 
other plants because of the slender aerial parts. Its ability to take up 
large amounts of water from the superficial soil must, however, be reck- 
oned with. There is little doubt that this is a dominating type. 

Rastrero (Opuntia megalarthra). 

This is a spreading, low form of prickly pear. Though sometimes 
very densely packed, making progress difficult, mechanically it interferes 
comparatively little with guayule. This is to be explained by the fact 
that, on account of the edgewise position of the flat, procumbent branches, 
very little soil surface is actually occupied. One finds, indeed, that young 
plants of guayule are frequently abundant in irregular rows beneath, or 
nearly so, the branches of the opuntia. It is not unlikely that the spines 
of the former aid somewhat in protecting the guayule from jack-rabbits 
and other predatory animals, and so, in this particular respect, help it 
along rather than hinder it. While this opuntia is a persistent type, its 
occupancy of the ground is apparent rather than real. 

A composite shrub (Zexmenia hrevifolia), huisache {Acacia farnesi- 
ana), gatuno {Acacia greggii), and asafran (Buddleia marruhiijolia) are all 
shrubby, freely branching kinds. The last resembles guayule in color, 
and the novice may easily mistake the one for the other. The gatuno 
and huisache are small trees with slender branches, and make but little 
shade. The nature of the competition between these forms and lechuguilla 
is more evident than in the case of these and guayule. They are slow- 
growing and do not reproduce themselves except by seed, and this not 
rapidly. Nevertheless, excepting the gatuno, they may be found growing 
very plentifully in some situations and often outnumber the guayule. 
Thus on north slopes the composite shrub is frequently more numerous 
than the guayule. 

Peyote (Peyotl) (Lophophora williamsii and L. lewinii). 

These cacti are the mescal-buttons or dry whisky of the Texas In- 
dians and cow-men, and have been sought after as the source of a little 
understood alkaloid of marked eftects upon the nervous system. The 
exposed part of the plant is little more than a convex disk a few centi- 
meters in diameter, of fleshy texture. The stem and root together form 
a conical, fleshy mass. They are a very modest element in the vegetation, 
occupying little surface, and may be disregarded from a practical point 
of view. 



40 



Guayule. 



There can be little doubt that the component elements in such a veg- 
etation are in a state of ebb and flow, and, in view of the density of the 
vegetation, in contrast with the condition usually met with in deserts, con- 
stitute an important question economically. Here the individuals come 
into actual contact above ground, where the competition is often severe, 
as well, presumably, as below ground. Referring especially to guayule, 
it may be accepted that, when a plant is once well started, it is seldom 
killed outright by contact with its neighbors, but the occupancy of the 
ground by other species which have superior methods for spreading grad- 
ually reduces the available surface and water-supply for the guayule. This 
plant takes advantage of surface-water by means of its superficial roots 
and plants with which it is associated and which behave similarly (e.g., 
Jatropha spatulata) must come into severe competition with it in this 
regard. But, assuming that, for purposes of forestry, it is desirable to 
thin out other vegetation in order to favor the guayule, the question 
arises as to the effect upon the germination of seed of this plant, which is 
undoubtedly favored by partial shade. It may be argued that the superior 
numbers of seed available and the shade of the guayule plants themselves 
will suffice, and this seems probable. On the germination of seed in the 
open more will be said, based upon experimental evidence (Chapter IV). 
Denuded areas are under observation, and the future may be expected to 
bring exact observation to bear upon the practical question of the value 
of clearing land, as well as upon the theoretical aspect of the questions 
above stated. (See also Chapter IX.) 

PARASITISM. 

Of vegetable parasites affecting the guayule only two are at present 

known. Of lesser importance, so far as we may judge, is a rust hitherto 

known as Uredo parthenii Speg. (fig. 5). Prof. J. C. Arthur, to whom 

material was sent for identification in April, 1908, reports that the fungus 

properly belongs in the genus Puccinia, and may be 

called Puccinia parthenii (Speg.) Arthur, ined., for the 

^ ^ (( ^ purpose of record. 

^^ ^-J ^ ^' It has been noticed that the fungus appears 

chiefly on plants which are on the north slopes of ar- 
royos, especially near the bottom, where the relative 
humidity is most favorable, since it is here that the 
highest vapor-tension exists. It has been found also 
on plants growing on ridges, and especially on those 
which are subject to a condition which we have called 
"witches' broom," in which the leaves are small and 
very much crowded. It appeared in the spring of 
1908 also on plants which had been grown under 
irrigation at Cedros, apparently on the older leaves, 
which still remained attached from the previous 
year. The parasite is not at all plentiful, and appears 
to be absent almost entirely from guayule growing in open situations.' 

* A small seedling which germinated in the early summer of 1908 was found 
in April 1909 with a single infection spot, quite in the open foot-slope (Station 3), 
in which situation the fungus is seldom seen. 




Fig. 5. — Teleuto and ure- 
dospores of Puccinia 
parthenii (Speg.) Ar- 
thur. 



The Environment. 41 

Of more importance, economically, is the "seda" (silk) or dodder 
{Cuscnta sp.), which often grows very plentifully. The habit of this par- 
asite is well known, so that no account of the plant is here necessary. It 
is very readily recognized as a yellow or orange vine-like leafless organ- 
ism which winds about the upper twigs and leaves of the host. It is not 
confined to the guaj^ule, being found also on hojasen (Flourensia cernua), 
on mariola {Parthenium incanuni), on tatalencho {Gymnosperma corym- 
hosum), and other perennial plants, and probably on some summer an- 
nuals. It reproduces itself by means of seeds which germinate after the 
advent of the summer rains, but is to be found vegetating vigorously long 
before this time. This is explained by the fact that it passes the win- 
ter in rather tight, compact clusters of thread-like stems, tightly wound 
about the uppermost twigs and leaves of the host. (Lloyd, ipoSd.) Thus 
it is independent of seed and is a true perennial.* 

The effect of the dodder upon the guayule is due to two causes. 
It diverts water and foods from the host into its own tissues and thus 
reduces the rate of growth, and it strangulates the twig and leaves upon 
which it fastens itself. There is thus produced a dwarfing and distortion 
which is reflected in the whole habit of the plant. 

As soon as growth commences in the host, the dodder, which is 
ready at the top of the previous year's growth to take hold of the new 
tender tissues, begins to twine about the newly forming stem and leaves 
and soon overtakes and strangulates them. The effect is to produce very 
slowly growing plants, and it is seen that the presence of much dodder 
would materially reduce the annual accretion of growth and therefore of 
rubber. In periods of severe drought the effect of the dodder is even more 
marked, since it diverts the already meager water-supply and thus causes 
the death of the portion of the twig at and above the zone at which the 
dodder is found. Plants with twigs killed in this way, and in which 
the dodder itself had succumbed, were found at the close of a sustained 
drought, in April 1909. The dodder should therefore be stamped out 
wherever it may be found. The best and only practical means is to har- 
vest with the initial crop all the guayule affected with the parasite. In 
this way the parasite will be checked, and additional checks will be re- 
ceived at each harvesting by following the same rule. 

Indications of another vegetable parasite were thought to be seen in 
the "witches' broom" above mentioned, but material examined by Prof. 
W. G. Farlow gave no clue to the cause. The densely packed leaves in- 
deed favor the growth of the rust already described, but this is quite a 
secondary condition. It is possible that the distortion is due to the crop- 
ping of the guayule by animals, but not all plants so treated show it, else 
nearly all would be affected. Plants closely in the field trimmed back 
(Station 2, quadrats i, 2) show a tendency to produce "witches' broom," 
indicating that constant or close browsing by animals may after all be 
the cause of this condition. 

' Cuscuia is sometimes a perennial as far north as the State of New York. 
Stewart et al.. Bull. 305, Agri. Exp. Sta. N. Y., Nov., 1908. 



42 Guayule. 

ANIMAL PARASITES. 

The root-system, particularly the tap-root and its larger branches, 
are frequently found to be infested with two species of the Coccidae,' Cero- 
puto yucccB (Coq.), and a species of Orthezia, distinguishable from the for- 
mer by the fluted, waxy egg-case attached to the abdomen. The number 
of these insects found on plants in the field is not inconsiderable, and may 
be responsible for lesions in the root-tissues which affect the growth of 
the plant. But of more importance is the circumstance that they occur 
in greater numbers upon seedlings raised under cultural conditions in 
wooden trays. Plantlets a few centimeters in height have been found 
with a dozen or more large individuals on the tap-root, the diameter of 
which was not as great as the breadth of the mature insects. They may 
therefore easily be responsible for retardation of growth, though external 
evidence of lesions has not been noted. 

Field plants especially are often infested below the surface of the 
soil by a scale, identified by Dr. C. L. Marlatt as Targionia dearnessi Ckll. 
This is a widely distributed species in this country. Large tap-roots are 
frequently half covered by this parasite. 

A gall insect attacks the leaves and inflorescence. The female punc- 
tures the young leaves and stems, the peduncles, and even the bracts of 
the capitula, and the resulting galls produce marked distortion. Many of 
the affected leaves fail of anything approaching normal development ; the 
peduncles are hypertrophied unevenly and become very much contorted, 
and the inflorescence fails to develop. The net result of the work of this 
insect is to reduce the rate of growth very materially and to cause a prac- 
tically complete abortion of the flowers and, therefore, of the seed. The 
plants affected are readily recognized on account of the irregularity and 
lumpiness of the terminal growths. The stems proper do not seem to be 
affected, as the insect appears to commence its work toward the close 
of the season of growth and to confine itself to the last-formed leaves, 
which remain attached throughout the winter, and to the inclosed young 
inflorescences. The increase of growth in the stem is, however, affected 
indirectly, and the annual accretions frequently amount to less than i 
cm., and scarcely ever to more than 2 cm., during the period of attack. 
Many plants in circumscribed areas are subject to the attacks of these 
insects, and it may readily become a serious menace to both the growth 
of the plants and to their seeding power. The following notes have been 
kindly furnished me by Dr. Mel. T. Cook: 

The study of this material presented many difficulties, as must necessarily be 
the case when it is not possible to make a field study. 

A gall produced by Cecidomyia parthenicola on Parthenium ^ in New Mexico has 
been described by T. D. A. Cockerell in Entomologist, July, 1900, p. 201. The gall 
before me does not fully correspond with Cockerell's species, and yet I should hesi- 
tate to say that it is an entirely different species without further study, which is im- 
possible with the material in hand. Dissection of the material showed two entirely 
different species of larva and immature insects, cecidomyid and cynipidous, while a 
study of the histology presented certain confusing and anomalous characters. 

* Kindly determined for me by Mr. J. G. Sanders, through the courtesy of 
Dr. L. O. Howard. 

^ Parthenium incanum, presumably. 



The Environment. 



43 



The isolated galls were small, monothalamous, and in the shape of a truncated 
cone, usually on the upper surface of the leaves and standing in an oblique position 
The opening of the larval chamber was through the top and was guarded by hair- 
like growths or trichomes, which pointed inward. This would indicate a cecidomyid 
gall, but certain preparations showed the opening closed by a thin membrane. 
Whether this latter condition was real, therefore proving the presence of two species 
of galls, or only apparent, was difficult to determine, owing to a tendency of the galls 
to coalesce, forming irregular masses. 

HISTOLOGY. 

The gall in its earliest state shows the reduction of the palisade into cells of 
the mesophyll type. This condition is characteristic of the origin of all leaf galls. 
As the gall develops, the cells, which constitute the lining of the larval chamber, are 
rich in protoplasmic content, which decreases from inner to outer surface. This is 
indicated very readily by the stains and is characteristic of the more highly devel- 
oped galls and usually designated as the nutritive zone. A little later certain galls 
showed a reduction of the nutritive zone and the formation of a protective zone of 
sclerenchyma cells just outside the nutritive zone. The presence of this protective 
zone is characteristic of the galls produced by cynipidous insects, and the 'viTiter has 
never found them in galls caused by cecidomyid insects. 

From the above facts, it appears that we may have two species of galls, one pro- 
duced by a cynipidous insect and the other by a cecidomyid, or a single gall which 
has been parasitized. 





.'krA 



Fig. 6. — The guayule barkbeetle {Pityophthorus mgricans Bland), (a) Work of beetles and larvae in 
barkandwood. (5) .^dult beetle, greatly enlarged. Small figure at right shows natural size, (c) Egg- 
galleries of parent beetles, with intervening larval mines, all grooved on surface of wood. (From 
illustrations loaned by the Bureau of Entomology. U. S. Dep. Agric.) 



44 Guayule. 

THE GUAYULE BORER. 

In the fall of 1907 it was noticed that guayule in the stack (plate 
4, fig. A), awaiting treatment for the extraction of the rubber, was being 
attacked by an insect, the only signs of which were the finely -powdered 
debris escaping from minute, circular openings in the bark. It was at 
once evident that a borer of some kind was at work. Material was sent 
to Dr. L. O. Howard, who kindly referred the matter to Dr. A. D. Hopkins, 
in charge of forest insect investigations. Bureau of Entomology, U. S. 
Department of Agriculture, to whom I am indebted for the accompanying 
notes and drawings (fig. 6, p. 43). Dr. Hopkins writes as follows: 

The beetle is Pityophthorus nigricans Bland. It has also been reported to the 
Bureau of Entomology by H. Pittier, who found it injuring the same plant at Tor- 
reon, Coahuila, Mexico. The insect is of special interest because of its habit of attack- 
ing a plant of such commercial value, and on account of its being the largest repre- 
sentative of the division of the genus to which it belongs. Those of one division 
infest coniferous trees only, while those of the other, to which this species belongs, 
infest only the broad-leaved plants and trees. The guayule barkbeetle evidently 
attacks the plant after it is dead, or soon after it has been cut, and, as has been 
shown by the specimens in the forest-insect collection of the Bureau of Entomology, 
may continue to breed in the same bark and wood for several years. It is evident 
that the prompt utilization of the plant for the manufacture of rubber within a few 
days after it is cut would prevent all losses from this source. 

Inasmuch as the buyers of shrub sometimes accumulate large quan- 
tities and place it in stacks until needed, and as this may represent large 
investments, the amount of damage may represent no inconsiderable 
loss. In order to determine what this loss might amount to, a piece of 
stem of average thickness which had been attacked by the borer was 
weighed as a whole. It was then decorticated and the insect d6bris was 
carefully removed. Some of the debris had of course been lost, and thus 
an error is introduced into the calculation of fully 5 per cent of the total 
weight of the bark. The tunneling done by the insect was not complete, 
however, and for this reason the figures may be regarded as the average 
result of the damage which may occur in the space of a month or two. 



Table 19. 



Length of sample piece of stem ....25. 

Diameter of the wood 9.8 

Thickness of the bark 2.3 

Total diameter of the stem 14.4 



grams 

Weight of the whole 3.801 

Weight of wood cleaned of debris . 1.703 

Weight of bark cleaned of debris . i . 903 
Weight of material lost (with 

probable correction) 0.2 

Of which half is bark, viz o . i 



It can be mathematically shown that the amount of destruction in 
the smaller twigs in which the insects work may amount to very con- 
siderably more, indeed to the extent of 40 per cent of the volume of the 
bark (cortex). Inasmuch as the bark contains practically all the rubber, 
it is seen that the loss may be great enough to warrant serious considera- 
tion. It must be observed, however, that the comminution of the cor- 
tical tissues by the beetle does not diminish the amount of rubber in 
the stem except by the amount that happens to escape through the en- 
trances, so that the real question is, whether the comminution of the cor- 



The Environment. 45 

tex and of the rubber contained in it renders the rubber unavailable in 
the manufacture of the crude product or not. In order to answer this 
question, a sufficient quantity of the debris was collected and subjected 
to mastication. By this means it was possible to cause the partial agglom- 
eration of the rubber, but it was quite impossible to separate out the 
"bagasse" on account of the fineness of the particles. These have the 
effect of separating the rubber so that it is in the form of a fine mesh- 
work, the connecting isthmuses not appearing to be great enough to 
overcome the surface-tension of the smaller masses. Microscopical ex- 
amination shows the mass to be composed of minute fragments of tissue 
derived from the wood and cortex embedded in the rubber. Measure- 
ments of these particles showed them to be 0.02 to o.i mm. in size, occa- 
sional pieces being as large as 0.5 mm. If during mastication one is 
careful to allow only a small amount of saliva to bathe the mass, it may 
be held together for some time, but if it be flooded for a moment and 
worked meanwhile, it will quickly disintegrate and can not be reagglom- 
erated. It therefore appears that the work of the beetle, while not destroy- 
ing the rubber, puts it into such condition that it is lost to the manu- 
facturer who uses a mechanical method of extraction, since the minute 
particles can not be made to agglomerate. When the insects have once 
got a fair sts^t in a stack-yard the amount of damage which may be 
caused in a short time by their very large numbers may be great enough 
to warrant the adoption of means to avoid the loss, if it is found that 
stacking the guayule is necessary. 

CROPPING BY GRAZING ANIMALS. 

It has been pointed out that the growing guayule is browsed by an- 
imals. Burros, jack-rabbits, cotton-tails, and goats are all given to this, 
and as these animals are numerous a great loss is entailed. Goats are 
herded habitually in the guayule fields, and these animals, with their 
all-devouring appetites, eat almost everything that grows. Not the least 
damage done by them is the wholesale destruction of the developing 
shoots and flower-buds, reducing the crop of seeds very greatly. Goats 
and burros may, however, be pastured away from the guayule fields, and 
thus loss may be avoided. 

The work of rabbits, where other food is available, is not serious, 
though in the event of adopting forestry methods they may become a 
menace to the plant. These marauders do not merely crop off the foHage 
and new shoots; they lop off whole branches, which are left on the ground 
to die. One jack-rabbit may therefore do a great deal more damage than 
a goat in the same time. It has been noticed that they treat the gober 
nadora in the same way. One frequently sees a complete circle of dead 
branches about the base of a bush, all having been lopped off at one time. 



CHAPTER III. 



DESCRIPTION OF THE GUAYULE, 

Partheniuni argentatum Gray. 

SEED. 
The word seed is here applied in a loose sense, inasmuch as the body 
to which the term is applied is, correctly speaking, an achene, a one- 
seeded fruit in which the pericarp remains indehiscent and dry. What 
passes as "seed" in guayule is a mixture of achenes, sterile flowers, 
involucral scales, and pedicels, and, inasmuch as the opportunities for 
sophistication are nearly always at hand, and for the reason that the 
peon employed for the gathering of seed will not always be diligent in 
distinguishing between guayule and mariola "seed," the present chapter 
may appropriately begin with a description of the flower. 

In the genus Partheniiim, as in all the Compositae, the order to which 
it belongs, the flowers are arranged in heads or capitula (fig. lo). In the 

guayule these are about 5 mm. in 
diameter, and contain two kinds 
of flowers, commonly known as 
ray and disk flowers. The rays 
are normally five in number, and 
are readily recognized during 
flowering by the open corollas, 
which project radially beyond 
the margin of the capitulum. 
These only produce seed, each a 
single one, if fertile.^ The disk 
flowers produce pollen but are 
incapable of setting seed, al- 
though the pistil is present and 
serves after the fashion of a pis- 
ton to eject the pollen, as commonly occurs in the Compositae. When the 
fruit is ripe and the period of flowering is quite past, the capitulum 
becomes dismembered in a somewhat unusual fashion. Each ray-flower, 
the two adjacent disk flowers and their subtending involucral bracts, 
become attached to each other by concrescence, and fall away as a whole 
(fig. 7). The remainder, i.e., all but ten of the disk flowers, also remain 
attached to each other and fall away as a shriveled, conical mass. There 
remain behind five involucral bracts persistently attached to the recep- 
tacle which supported the whole. In collecting "seed" all of these are 
taken, so that it will be seen that the bulk of the material is chaff. 

Considering the fertile flower and its accompaniments, we observe 
that the achene is hidden between the adjoined pair of disk flowers and 
its own bract. This bract, which is quite broad and concavo-convex, is 




Fig. 7. — Ray- flower with attached disk flowers and 
the subtending bracts. Parthenium incanum. 



* Polyembryony occasionally occurs. 



46 



Description of the Guayule. 



47 



composed of three morphological elements, fused above, but more or less 
loosely connected below; a rare occurrence, analogous to the condition 
of some stamens. The middle element 
is the narrowest, and is the bract proper 
of the pistillate flower. This, to be seen , 
must be dissected out. 

Another peculiar feature then be- 
comes apparent, namely, that the two 
disk flowers can not be separated from 
the achene without pulling away two 
narrow strips of tissue from its margins. 
(Fron et Francois, 1901.) The whole 
arrangement would appeal to the tele- 
ologist as an excellent adaptation for 
dissemination by the wind or by water, 
since the thin, light, and air-imprison- 
ing tissue may serve as wings or floats 
according to circumstances. The achene 
itself is crowned by the persistent but 
shriveled corolla, and at either side of 
this and against its ventral (upper or 
inner) aspect are three short awns,' one 
in each position. The achene proper 
is ovate, with an acute base. It is par- 
tially clothed with short appressed hairs, 
but for which the pericarp would be 
black or dark gray. The achene meas- 
ures 2.5 mm. in length by 1.8 in breadth 
when of normal size, exclusive of the 
awns. 

The "seed" of the two other spe- 
cies, mariola (P. incanum) and P. hys- 
ierophorus^ (an annual), which grows 
with or near the guayule, may be 
distinguished by attention to the char- 
acter of the lateral awns, which may 
readily be seen with a lens by viewing 
them as they project beyond the bract. 
In the guayule the awns are brown, 
with papery, denticulate margins. In 
the mariola these are slender, appearing 
denticulate or quite without membra- 
nous margins, tapering and distinctly 
refiexly curved, and are usually darker in color, being black toward the 

' Taxonomic works u.sually indicate that there are only 2 awns, but this is 
an error. There are 3 awns in Parthenium argentatum and P. incanum; 2 in the 
herbaceous P. lyratnm and P. hysterophorns. Engler and Prantl describe the genus 
as having 2 to 3 awns, but do not indicate further details. 

' This plant grows in great profusion in the summer months in the alluvial 
plains upon which the guayule lands border. 




Fig. 8. — A, a fully germinated seedling of 
guayule, before induration, X 8 ; B, cotyle- 
dons and first two foliage leaves ; C trans- 
verse section through achene of a ray flower 
and its two attached disk flowers. 



48 



Guayule. 



base. In P. hysterophorus and P. lyratum they are very broad, and are 
membranous in the former. Figs. 7 and 9 will make these and other 
characters evident. 




Fig. 9. — Achenes of (i) Parthenium argentatum, (2) P. hysterophorus, (3) P. incanutn, (4) P. lyratum. 

SEEDLING. 

When germination is complete the seedling of the guayule consists 
of a short primary stem (hypocotyl), 5 to lo mm. in length, terminating 
in a long, slender tap-root. Attempts to find the end of this in the field 
have been fruitless, on account of the nature of the ground and because 
of its very tender character and great length and thinness. Experiments 
show that it reaches a depth of at least several inches. This slender root- 
let, with very few branches, is the means of keeping the plantlet suppHed 
with water from the soil for some months, as frequently during the first 
year in the field no adequate development of lateral roots occurs. The 
seed leaves (cotyledons) are nearly or entirely circular in form, and range 
in size from 2.5 mm. in width by 3 mm. in length to 4.5 mm. in width 
and 4.7 mm. in length, according to various conditions. At the apex of 
each cotyledon is a hydathode, composed of a group of water stomata. 
Other conditions being the same, seedlings grown in the shade and high 
relative humidity have the largest cotyledons (plate 34, figs. 6, 9), and 
the largest were seen on seedlings grown experimentally under such con- 
ditions. The primary stem is about i mm. in diameter, and in seedlings 
grown under natural conditions, i.e., with direct sunlight, is dark red; in 





A. The root-system of guayule. 

B. Group of plants which started as retonos. 

C. A strongly monopodia! retofio. 



Description of the Guayule. 49 

shade plants it is green. The dark-red color extends also over the under 
surface of the cotyledons, which are rather thick in sun forms, and thin- 
ner in shade-grown plantlets (plate 34, figs. 4, 6). 

The early foliage leaves, soon after germination and because of the 
very short intemodes, are closely crowded. By partial etiolation these 
intemodes may be caused to lengthen, and thus the structure of the pri- 
mary epicotyledonary stem may be better studied. In this way points 
may be made clear which otherwise would with difficulty be explained. 
The first 8 leaves are usually ovate, entire, slightly acute, and taper 
suddenly into the petiole (fig. 8). They are clothed, as are all the foliage 
leaves, with closely set T-shaped hairs (plate 30, figs. 9-1 1) laid parallel 
to the axis of the leaf, and thus is produced that light green-gray, satiny 
sheen which characterizes the plant. The first leaf is usually about i cm. 
long by 3 mm. wide, though measurements vary a good deal. In the mar- 
iola seedling the earliest leaves resemble those of the guayule, but differ in 
being broader and lanose, a difference due to the form of the trichomes, 
those of mariola being of the "whip" form found frequently in the Com- 
positae. As the hairs are much thicker on the under side of the leaf, the 
species may be very readily recognized even when only one foliage leaf 
has developed, though identification is difficult before this leaf appears. 

The last formed of the entire-margined seedling leaves may reach, 
in field plants, a length of 7 to 8 cm. and a width of 1.5 cm. The first 
approach to the mature leaf form is seen in a single tooth, usually on 
one margin only, at about the middle of the blade. In the next stage 
the tooth may be found on both sides, and larger, while half-way between 
their position and the apex a second pair of teeth appears. By basal 
contraction of the blade and extension of the upper portion, the first 
teeth appear to move downwards, and by enlarging attain lobate pro- 
portions. The leaf is now relatively shorter and broader. An additional 
pair of basal teeth may also add to the complexity. While this descrip- 
tion, illustrated well in plate 18, is generally true, few plants are more 
variable as regards the form of the leaf than the guayule, and this varia- 
bility is, with the exception of the earliest foilage leaves to be formed, 
closely connected with the amount of available soil-water. Thus we find 
that in plants grown under irrigation the amount of lobing is very much 
more marked than in field plants. We shall return to this subject later. 

The first inflorescence is usually formed early in the history of the 
plant, and may occur in the first growing season even in field plants, 
though this is exceptional (plate 17). This early flowering in a shrubby 
plant of long life appears to reflect its relationship to herbaceous forms, 
and would not improperly be regarded as indicating that the perennial 
habit of the guayule and mariola is, phylogenetically, a recently acquired 
character. The inflorescence, which is a compound monochasium (fig. 10) , 
is terminal, and thus ends the growth of the chief shoot. In some in- 
stances flowering may not occur for some years, and in this event if no 
accident befalls the chief shoot it may attain a length of 15 cm. or more 
before the first flower shoot appears to conclude the growth of the chief 
axis. In such a case the lateral shoots make but little growth. Upon 
the first occasion of flowering the growth of the branches begins; these 
4 



50 



Guayule. 



in turn terminate in inflorescences and, by ending their growth, give stim- 
ulus to the growth of branches of higher orders, each in its turn. Thus 
the plant becomes profusely branched, and this habit contributes mate- 
rially to the amount of secretion, which is proportional to the number of 
branches. 




Fig. 10. — The inflorescence of the guayule. 



THE MATURE PLANT. 



ROOT-SYSTEM. 
The root-system of the guayule consists, in a plant derived from a 
seedling, of a strong tap-root extending to a considerable but undeter- 
mined depth in the soil. The lower end, which branches more or less, 
draws upon the water-supply of the deeper layers of the soil, especially 
in younger plants. Just below the surface of the soil a number of strong 
lateral roots are given off, which in many instances are of extraordinary 
length, reaching a distance of 150 to 200 cm. or more from the plant 
(plate 9, fig. A). These serve to take up the water in the shallower lay- 
ers of the soil, derived from rains sufficient to wet the soil to this depth. 
Such far-reaching, shallow-placed roots are characteristic of many desert 
plants. Cannon (1909) has studied and mapped the root-systems of a 
number of such, and has further shown that competition between juxta- 
posed plants may be eliminated by the difference in the type of root- 
system, the one going deeply, while the other is chiefly shallow. The 
development of two differently placed parts of the same root -system, the 
one drawing on the deeper, the other on the shallower layers of the soil, is 
of very great importance biologically, and is well exemplified by the little 
cactus Ariocarpus kotschubeyanus , which grows in the alluvial plains of the 
mesa central. The shallow roots arise from the top of the tap-root and 
ascend as nearly vertically upward as may be, till they reach to within a 
few millimeters of the surface of the soil, when they suddenly take a hori- 



Description of the Guayule. 51 

zontal position, and in this direction traverse considerable distances from 
the plant. This condition is closely analogous to that in the guayule and 
serves to make even clearer the significance of the arrangement in that 
plant and in others, in all of which the tap-root system, while quantita- 
tively inferior both as regards the number of branches and the amount of 
water absorbed into the superficial system, may nevertheless be of a good 
deal of importance in enabling the plant to withstand prolonged drought 
when the shallower portions of the soil become very dry. This is indi- 
cated by the readiness with which retonos arise from the tap-root after 
the plant and lateral roots have been cut away. 

RETONOS. 

From these shallow lateral roots there frequently arise new adventi- 
tious shoots, sometimes singly, sometimes in groups of two or more (Lloyd, 
igoSc) (plate 9). They are locally called "retonos," though this term is 
not always used strictly , and may apply to shoots arising from stem tissues. 
It is the same word as " rattoon," used by sugar-cane planters, but as this 
is used constantly to indicate offshoots from the base of the stem, it is 
inapplicable as an equivalent of retono. Since it is the only term used in 
Mexico for these shoots of root origin and as our common EngHsh equivalent 
is characterized chiefly by its inelegance, we shall venture to retain the 
Mexican-Spanish expression. 

Retoilos usually arise from the plant at a distance of 20 cm. or more. 
They have been found at a meter's distance, and doubtless may occur 
still further away. The point of origin may be above, below, or at the 
side of the root. As growth proceeds the proximal part of the root fails 
of further secondary thickening, or at most undergoes very little thick- 
ening. It ultimately becomes abstricted by decay, apparently induced 
by pressure of the tissues of the retono, and quite soon loses its physio- 
logical value. The distal portion, however, thickens more rapidly, keep- 
ing pace with the growing retono, and takes on the proportions of a tap- 
root, though it may always be distinguished from a true tap-root by its 
curvature and position in the soil. Secondary, adventitious roots (fig. 11) 
later arise from the basal portion of the stem of the retono, thus amplify- 
ing the root-system. A large root-system thus developed is shown in the 
central and largest plant in plate 9, fig. B. 

The author of this publication stated as follows in a previous paper : 

The formation of these new plants in this manner is not spasmodic or excep- 
tional, nor are they fugitive in their nature. Under certain conditions they are pro- 
duced in such numbers as to entirely overshadow the numbers of seedlings ; and they 
as frequently grow into maturity, producing a plant which, if the origin were not 
known, would not unlikely be considered a varietal type, in point of habit. The 
mature plant which had its origin as a seedling has a single'trunk, usually 10 cm., 
sometimes 20 to 30 cm., in length; the mature plant produced vegetatively has usu- 
ally a very short trunk, or a group of separate ones, more or less coalesced by growth, 
though marked exceptions may occur (plate 9, fig. C). 

The ratio of the number of new plants arising as seedlings and of those arising 
as root-shoots varies with the habitat. Both forms may be found in any situation, 
but the retoiios are much more numerous on stony slopes, often outnumbering the 
seedlings. The reverse relation is seen in more level places. Thus, at the foot of a 
low ridge I have found seedlings plentiful, as many as 30 in a square foot (these 
small and larger ones as well scattered about relatively thickly). A zone of this 



52 



Guayule. 



character could be traced arovmd the ridge. Just above this zone another could be 
made out in which the retoiios were abundant and the seedlings scarce, while 
coining to the top of the ridge the seedhngs again outnumbered the retoiios. Thus 
on that part of the slope most affected by erosion, and where there is more chance 
of uncovering the shallow roots, the retonos are most abundant. It would appear, 
therefore, that the exposure to light is a potent, if not the most important, factor 
in inducing budding in the roots. Yet I have found that when a plant is removed 
by cutting at the base so as to sever the roots and leave them in the ground, shoots 
start from the root, not only where the root is accidentally exposed, but as far 
back as the drying out of the root makes it necessary. A root thus severed in 
January failed to bud till June in consequence of the lack of rain; when at last it 
rained, the buds started out 12 cm. away from the cut end and several centimeters 
deep in the soil. On the other hand, roots purposely exposed for a portion of their 
length and slightly wounded had failed to start buds at the end of six months when 
last examined.' So the case appears to be more complicated than at first appears. 
Injury may be a factor at times, but, experimentally, I have shown that scarring 
or cutting the cortex is not sufficient to insure budding, at least under field condi- 
tions, for it is probable 
that the exposure to a 
low relative humidity in- 
hibits the growth of callus 
on exposed roots. It is 
more probable that had 
roots been injured and 
left covered with soil, 
positive results would 
have accrued. 

This occurrence of 
retonos in guayule pre- 
sents a very interesting 
biological phenomenon. 
In a habitat where the 
rainfall is very meager, so 
that years occur in which 
the conditions for germi- 
nation are prohibitive, and where, moreover, sudden and severe rains wash the soil 
on the steeper slopes severely enough to remove seeds or expose seedlings when 
young so as to prevent their further growth, it will easily be seen that the vegetative 
method of reproduction presents certain very marked advantages. This is true also 
where the soil is confined to the crevices of the native rock where it lies at or very 
near the surface. This condition occurs very frequently in North Zacatecas, where 
large areas will be seen in which the vegetation is confined to bands of outcropping 
rock, where it occupies the soil beneath the edge of a stratum. Where the relation 
of the strata to the surface is such that flat blocks of rock support but a thin layer 
of soil, the distribution of vegetation is determined by the fissures. In the case of 
guayule we have an exception, for this plant may send out a shallow lateral root 
over a block of stone, above which plants may start. Very frequently we find 
individuals which have grown in this position, with their roots straddling the sub- 
imposed rock. Such are almost invariably retonos. Plant i (plate 9, fig. B), was 
found so placed. There are other plants which can compete with the guayule in 
this regard, such as the lechuguilla {Agave Iccheguilla), which spreads out by means 
of stolons, and occupies areas for itself to the exclusion of everything else. It is 
clear that the habit described is of no small importance in the fight for foothold. 
One can easily imagine, too, that a distinct advantage is to be had in the rate of 
growth and the quickness with which the ability to flower abundantly is reached 
by retonos. The rate of growth is relative to the size of the mother root ; but it is a 
very common thing foraretofio to grow 10 cm. and to come into flower in two months 
in summer (plate 9, fig. B, 10 and 11). Seedhngs, on the other hand, flower only 




Fig. II. — Retoflos, showing position of adventitious roots, pr., 
proximal portion, and dr., distal portion of mother- root. 



' Only negative results were had as late as September 1908. 




An exceptionally tall (130 cm.) individual. Weight 9.4 lbs. Caopas. 



PLATE 11 





A. A widely spreading (130 cm.) plant of guayule. Weight 10 lbs. 9 ( 

B. A large plant of the usual habit. Weight 8.5 lbs. Apizolaya. 



LLOYD 



PLATE 12 




Description of the Giiayule. 53 

infrequently before the third year, and the amount of growth then does not more than 
equal that of a single-stemmed retoiio in one year. At the end of three years the 
retono makes a considerable plant (6 in the same plate), and flowers richly. The 
influence which retoiios would have in reforesting processes, both by their own growth 
and by seedlings, can therefore be well appreciated, and probably with difficulty over- 
estimated. Some basis for judgment in this regard will reward a study of the accom- 
anying photograph ' (plate 9, fig. B, in which the horizontal lines are to be regarded 
as 10 cm. apart). 

From his comparative morphological and anatomical studies on " nor- 
mal" parts and those of individuals ("rejets") arising from root buds, 
Dubard (1903) draws the following conclusions: 

En r^sum^, la multiplication par bourgeons radicaux est un fait peu normal 
dans le regne v^g^tal; elle donne naissance a des rejets d'organisation infdrieure, dans 
la plupart des cas; chez quelques ^speces elle tend a s'^tablir d'une fa^on r^guliere, 
mais ne devient qu'exceptionnellement une sauvegarde effective de I'^spece. 

The inferior organization of the retonos studied by Dubard is always 
in the direction of an anterior form "by virtue of hereditary ante- 
cedents": "les rejets radicaux des diverse especes d'un meme genre 
manifestent une convergence qui ne peut etre fortuite." 

The retoiios of the guayule are in the same case. The absence of 
medullary and of cortical canals is a marked return to a more simple 
structure, as is also the absence of medullary stereome, in which, and in 
the absence of canals in the medulla, we see an assumption of seedling 
characters. But the retoiio assumes a still more ancient condition, we 
may believe, in the loss of the cortical canals. 

Nevertheless, the guayule, while in this measure conforming to the 
observations made by Dubard, can not on any account be relegated to 
a subnormal category, characterized by comparative impotence in safe- 
guarding the species. The frequently strong vegetative growth ; the early 
maturation of flowers and seeds ; the already established root-system ; the 
cincture of the mother root tending to separate the retofio physiologically, 
if not always structurally, from the parent plant (fig. 11); its frequently 
wide separation from this; its ability to gain a foothold where seedlings 
must surely perish; all these facts heighten the importance of the retono, 
despite the relatively small numbers in which they are found, in enabling 
the species to maintain a foothold. It seems, indeed, not unlikely that 
a further classification beyond that of Dubard will be necessary — one for 
those plants in which the retoiio is of great importance in this regard. 

A comparison at this point between the guayule and the mariola is 
of special interest, because, while they are closely related species, their 
methods of vegetative reproduction are quite distinct. 

In the first place, the root-system in the mariola differs in that the laterals 
run at a steeper angle into the soil. Occasionally retoiios are formed, but, as far 
as my observation goes, always close to the plant, within, say, 5 cm. What always 
happens, however, is this: From the basal portion of the stem, where there are 
many dormant buds, as a sequence of the short internodes marking the slow initial 
growth of the seedling, new, slender shoots arise, growing to a height of 30 cm., 
more or less, in two months. From the base of each such shoot an adventitious 
root starts out, immediately above the point of origin of the shoot. This usually 
single root develops as a tap-root, and supplies all the water for the daughter shoot, 

» F. E. Lloyd, 1908c. 



54 Guayide. 

which develops apace, and ultimately becomes an independent plant. The isthmus 
of tissue between it and the parent plant does not enlarge much in any case, so that 
it is quite easy, on taking up a bush of mariola, to separate it into several smaller 
plants by merely breaking off the functionally independent elements. Thus the 
habits of mariola and guayule in this regard are so different that one plant, the former, 
remains a single-stemmed shrub of tree-like habit, while the mariola is of the bushy 
habit. This marked difference, it will be seen, precludes the advisability, though 
the possibility might remain, of grafting the guayule on the mariola, a suggestion 
which has. been made on the assumption that increased growth might follow in the 
scion. No economic result would follow, and for this reason: Suppose that we suc- 
cessfully graft a piece of guayule on a stock of mariola. The scion grows, but at the 
same time new shoots arise from the base of the stock as described, and their growth 
is so rapid that in a month or two the guayule shoot is overtopped, and this ends the 
usefulness of the graft for economic purposes. We might very well make a graft 
for the purposes of pure science, but economically it would be a failure (Lloyd, 1908c) . 

Recently it has been proposed (Escobar, 1910), but with admirable 
reserve, that the dissemination of guayule seed in areas where only ma- 
riola grows may be attained by grafting guayule upon it. The plan ap- 
pears impracticable. 

METHOD OF BRANCHING. 

It has been pointed out above that the monopodial growth of the 
seedling is brought to a close by the development of the first inflores- 
cence. Following this event, several of the uppermost branches make 
a more rapid growth. These branches in turn end their growth each by 
the formation of an inflorescence, when usually the two or three upper- 
most buds continue to lengthen. Thus is produced a constantly divari- 
cating system of stems (plate 11, fig. A), which, if uninjured, results in 
a splendidly symmetrical and closely branched shrub. A very excep- 
tional plant, approaching the ideal form, is seen in plate 11. Through 
failure of some branches to develop, irregular forms are often seen. These 
usually attain a greater height than the symmetrical plants. An unusu- 
ally tall plant is shown in plate 10, in which the irregularity of growth 
is illustrated, while in plate 11, fig. B, a form more frequently met, espe- 
cially in very rich fields, is shown. 

A comparison with the mariola is here pertinent, as there appear 
to be two types of guayule in respect to the manner of branching, one of 
which approaches the condition in mariola. The usual manner of exten- 
sion of the branching system is by the nearly equal growth of two or three 
branches just below the inflorescence (plate 14, fig. B) . As will be seen, the 
anatomical distinction between stem and peduncle is abrupt, and the dead 
and, according to age, more or less disintegrated peduncle remains as a 
spur in the angle between the uppermost branches. Often this may still 
be seen after the lapse of many years. No absciss layer is formed/ and 
this again gives a suggestion of the recent departure of the shrubby type 
from the herbaceous ancestor. After flowering, the dead peduncles re- 
main in evidence above the foliage of the plant and form a conspicuous 
character. In the mariola, on the other hand, with the same morphologi- 
cal basis, a different habital form is had. The stem, as in guayule, ends 
in an inflorescence, is more slender, and is beset with short branches or 

' This condition is, of course, common to many plants, and is specially preva- 
lent among the Compositae. 



Description of the Guayule. 55 

spurs, which, because of the more rapid growth of the shoot in mariola, 
are more numerously developed. The transition into the peduncle is grad- 
ual, and not sudden, as in guayule; this organ is, therefore, not sharply 
delimited, either morphologically or anatomically, and is leafy and pro- 
vided with buds well up beneath the inflorescence. In the following grow- 
ing season, and this usually means in the following year, some of the 
short spurs develop into leafy branches and in their turn terminate in 
peduncles. These, like all the branches, are slender and tapering, and 
their position, rate, and manner of growth result in a close interweaving 
of stems, in striking contrast with the guayule. 

BIOTYPES. 

Returning to the subject of habital types in the guayule, it has been 
found that some plants have the mariola manner of growth (plates 12 
and 13). Instead of an abrupt termination of the stem at the base of the 
peduncle, the transition is gradual and the stems are of smaller diameter 
than in the usual type. Foliar differences are to be noted beyond. The 
matter is possibly of practical importance, as the slender branches with 
vaguely delimited flower-stalks would, mutatis mutandis, contribute to pro- 
duce a less desirable form of plant from the point of view of production. 
A phylogenetic interest also attaches to it, inasmuch as the mariola habit 
is more closely comparable to the herbaceous manner of growth, as dis- 
played by congeneric herbaceous species, than is the guayule habit. On 
this score, as on others, the guayule is the type more widely divergent 
from the theoretical herbaceous ancestor. 

These differences are, indeed, quite fundamental, and may be traced 
back to the earliest seedling stages (plate 13). The clearness of the dis- 
tinctions is such as to indicate that we are dealing with a field mutant, 
and the differences in the structure of the awns (pappus) would seem suffi- 
cient ground, in the light of the taxonomy of the genus, to warrant us in 
regarding the broad-leafed type as a distinct species. The two forms, 
P. argentatum proper and this closely related form, be it a well-marked 
species or a type of less taxonomic evaluation, are remarkably distinct, 
and call to mind many similar juxtapositions of closely related species, 
recognized as such, known to occur among plants, but not yet properly 
appreciated as evidence in the discussion of isolation (Lloyd, 19056). 

Another difference in the habit — though not correlated, it appears, 
with the manner of development of the inflorescence — is seen in what 
may be termed straight and crooked limbed forms. The one is clean- 
cut and smooth-limbed, each span of growth being nearly straight; the 
other is rougher barked, the more slender limbs showing marked curva- 
tures. The former is the more rapidly growing type, suggesting differ- 
ences in the available water-supply. One frequently finds examples of 
very marked growth differences in field plants, such as are shown in 
plate 9, fig. A, of which the right-hand plant grew in a shallow rock crev- 
ice and was unable to develop a competent root-system. The annual 
accretions of growth in this plant were very short, not exceeding a centi- 
meter, and this resulted in the production of a very dense, much-branched 
mass of limbs, as seen in plate 9, fig. A, on the extreme right. This and 



56 Guayule. 

the left-hand plant in the same figure show extremes of rate of growth, 
somewhere between which lies the average, which it is desirable to know 
in estimating the rate of reproduction. 

A still further difference in habit, which is not very readily distin- 
guished from the foregoing at first glance, is one recognized by persons 
engaged in the gathering of the shrub, who designate the two types in 
question "macho "or male and "hembra" or female. The differences, which 
are shown in plate 14, fig. B, were pointed out to me by Don Jose Herrera, 
a gentleman who has had a great deal of practical experience in collect- 
ing shrub. " Macho " guayule has fewer branches, and they have a larger 
diameter than those of the "hembra," in which the branches are much 
more numerous. These terms are not here used in the sense spoken of on 
page 4, to distinguish guayule from mariola, which latter is sometimes 
called "hembra de guayule," but merely to designate the plant with the 
stronger and therefore "macho " habit and that with the weaker or "hem- 
bra" habit. These adjectives are used analogously with respect to other 
plants showing similar differences. " Hembra " guayule makes greater bulk 
when made up into bales, and for this reason those who gather shrub pre- 
fer to take it if they are being paid at a rate per bale. Whether the dif- 
ferences are biotypic or are due merely to environmental conditions can 
not be said; nor whether there are other correlated differences, as in the 
amount of rubber secreted, though such are variously claimed to obtain. 
There appears to be a stronger tendency in the " hembra " for the branches 
to run out into inflorescences, entailing a greater amount of dying back 
at the close of each growing-season, and thus it may turn out that these 
differences are essentially the same as those mentioned previously. 

Finally, many guayule gatherers and others think to recognize dif- 
ferent kinds as to color-characters, either of the bark or of the leaves. In 
Durango white guayule ("bianco") is distinguished from dark or "prieto," 
though no other characters could be pointed out to separate the two 
kinds. Indeed, when a branch was exposed to view in one position, so that 
the under surface of the twigs was seen, it was pronounced "prieto," 
and when the upper surface of the same branch was later shown it was 
called "bianco." This color difference, as between the upper and lower 
surfaces of the branches, is quite constant. 

"Blanco" and "ceniso" or ashy guayule are maintained to be dif- 
ferent also, though the same difficulty of seizing upon other than mere 
color differences obtains. So far as I could determine, "ceniso" guayule 
was shrub which had been exposed to severer drought, in consequence 
of shallower soil in exposed positions, as on benches, and in which the 
leaves had therefore dried to a dirty-yellowish color. Prolonged study 
might, however, discover that some of these differences are constant and 
racial, and the matter therefore deserves more consideration. 

SIZE. 

The question is often asked, especially by persons interested from 
the business point of view, as to the size which the guayule attains. It 
may at once be said that anything like the maximum size is a matter, 
or will be shortly, of academic rather than economic interest. Once the 



PLATE 1 3 




i 



'^Sa. 



■>. T' 




5 cm. 

I : 1 




LLOYD 



PLATE 14 





A. An irrigated plant, from a small stock, at the height of flowering. 

B. "Embra" (on the left) and "Macho" (on the right) guayule. 



Description of the Guayule. 57 

virgin guayule has been removed, big plants will no more be seen. The 
largest plants which have been reported weighed in the neighborhood of 
lo kilograms. Overseers of field experience insist that they have seen 
and weighed such. Endlich (1905) quotes Marse as having seen a plant 
weighing 6.5 kilos; but a plant weighing over 5 kilos is exceptional. Of 
three large plants which are illustrated in this paper, that in plate 12, 
fig. A, weighed 10.56 pounds (fresh weight), was 75 cm. tall, and 125 cm. 
wide. Plants over a meter in height are seldom met with, and are nearly 
always more or less stag-headed and moribund (plate 10). They have 
usually lost a good many limbs, and for many years have not been mak- 
ing any net gain in weight. Endlich places the average weight of virgin 
guayule at 500 or 600 grams. As will develop in the discussion in the 
following chapter, plants of this size, which would be 40 to 50 cm. or 
more tall, will in the future be considered large plants. 

SURFACE CHARACTERS OF THE STEM AND METHOD 
OF DETERMINING AGE. 

The importance and difficulty of determining accurately the age of a 
particular guayule plant has prompted careful study of the appearance of 
the surface of the stem at various ages (plate 14, fig. B). This appearance 
is due to (i) the primary superficial characters (epidermis, leaf-scars) and 
(2) the succeeding secondary cork. Secondary changes in the cork are 
produced by weathering. As marks also aiding in the determination of 
age may be mentioned the dead but persistent peduncles and the number 
of divarications of the stem, as related to the formation of inflorescences. 
Data relating to the rate of growth of seedlings, the marks of which are 
usually quite obliterated in plants taller than 10 or 15 cm., must also be 
considered. 

FIELD PLANTS. 

Let us suppose that we are examining a plant at the close of the 
growing season of, say, 1908. The characters seen in the accretions for 
the years mentioned will be as follows : 

1908. Leaves still adherent. The epidermis is intact and densely 
clothed with appressed T-shaped hairs, producing the greenish-gray color 
uniform with the leaves. If the length of the year's growth is exceptional, 
say above 10 cm., the basal part may show slight longitudinal fissures. 
Diameter at base 3 mm. or less, rarely more. 

1907. Epidermis still adherent, but more or less fissured, showing 
yellow cork. The hairs have been partially removed by attrition and 
withering, but most of them remain, preserving a gray color. Epidermis 
light brown. Leafless, but scars present. Often with short spurs, or un- 
developed branches with each a few leaves. Diameter usually between 3 
and 4 mm. 

1906. Color gray, slightly slaty brownish, generally fissured, the fis- 
sures shallow, disclosing a gray-colored cork (weathered) , with small areas 
of epidermis remaining between. Diameter about 5 mm. 

1905. The growth for this and earlier years is dark gray, becoming 
darker with age. The fissures are shallow, becoming deep only with an 
age of over 10 years. The fissuring is deeper, and lenticels are more abun- 



58 Guayule. 

dant on the lower surface of stems which are not in a vertical position. 
This is because of the thicker development of bark on this side. On old 
stems the fissures attain a depth of a few millimeters and become long. On 
very old stems the base may become transversely fissured also (plate lo). 

In using the above marks as a means of judging the age of a plant, 
one may with considerable accuracy judge of the amount of growth for 
3 or 4 years, and the average of these will come ver}^ near to the truth. 
Some difficulty may be experienced as the result of reduplicated growth 
in one year confusing the evidence offered by the leaf-scars, which are 
crowded fairly closely in the region where the intemodes of the winter 
buds occur. These are of the tropical type, there being no specialized 
scale -leaves, and consist merely of a few terminal leaves of small size 
which persist till the following season of growth. 

The natural wounding which results in fissures, especially as the stem 
grows older, as well as the accidental wounding which frequently occurs, 
usually sets free more or less of the resin, ^ of which large amounts are found 
in the cortex, as in the pith. The escaping resin collects as drops on the 
wound and, as it increases in amount, falls on the ground. Under every 
guayule plant of any size, therefore, a good deal of resin in the form of 
limpid masses of irregular size may be found. Should it turn out that the 
resin is of particular value (Chute, 1 909) , as for a special varnish, consider- 
able amounts could be collected by peons. 

IRRIGATED PLANTS. 
In irrigated plants secondary thickening begins within a short dis- 
tance (5 to 15 mm.) of the growing-point, and proceeds at a rapid rate. 
The fissures are very long and straight, and long patches of epidermis 
are left which may be still visible 30 to 40 cm. from the apex. The color 
for two years remains a clean, pale yellow, modified by the gray of the 
adherent hairs wherever patches of epidermis remain (plate 2 1 , fig. A) . 
The diameter, which remains nearly the same throughout the length of 
a year's growth in a field plant, making the growth cylindrical, increases 
rapidly in irrigated plants, so that the basal diameter may be three times 
that of the tip in the first year and eight times at the end of the second 
year. The early fissuring and the coloring are correlated with this rapid 
secondary thickening. 

THE LEAVES. 

The leaves of seedlings have already been described. In the adult 
plant the form of the leaf varies according to the amount of water avail- 
able and its position on the twig. In general the water-factor determines 
the amount of lobing. This is apparent in field plants as well as in those 
grown under irrigation, and the relation is made manifest, in field forms 
especially, in the sequence of leaf-form seen during the growing and the 
subsequent resting period, consequent on drought and cooler tempera- 
tures. The guayule may be called semi-deciduous, as it sheds a part of 
the leaves only, namely, those which are produced between the more elon- 

^ Loss of resin by secondary thickening is for the most part prevented by 
plugging of the resin-canals (Chapter V) . 



Description of the Guayule. 59 

gated internodes. Those which are still crowded together in the terminal 
bud-cluster remain and form the basal leaves of the subsequent season's 
growth. These leaves are the last to be developed, that is, at the close of 
the growing-season. Since the length of the season is determined chiefly 
by the decrease of soil-water, the shape of these last-formed leaves seems 
to be conditioned by this circumstance. This is evidenced by the fact 
that irrigated plants, to which water is available, continue to form lobed 
leaves (plate 21), and even those which compose the terminal bud are, in 
some plants, as deeply lobed as the rest. 

The winter leaves, as we may call those which persist in the terminal 
bud, are from i to 3 cm. long by 3 to 7 mm. broad, elongate-ovate, taper- 
ing into the petiole, entire, or with one or two very much reduced teeth, 
acute. The summer leaves are 6 to 7 cm. long by 2 to 2.5 broad when full- 
sized, and are deeply lobed midway the length of the blade. A large 
amount of variation is met with in these leaves, however, the form depart- 
ing from the proportion given to a long, slender, merely toothed leaf, 7 by 
0.7 cm. The summer leaves persist, in field plants, till December or later, 
at which time they begin to fall. By the middle of February all the leaves 
excepting the terminal bud-leaves have fallen, leaving the gray twigs bare, 
each surmounted by its leaf-cluster (plate 14, fig. B). Leaf-fall appears 
to be a function of drought rather than temperature. Long before falling 
the leaves show marked shriveling and curling, and fall away as much by 
drying ofE as by the action of an absciss layer (see Chapter V) , which is 
imperfectly formed. In irrigated plants leaf -fall is much less prompt, 
proceeding from the base of the previous season's growth upward, the pro- 
cess not being completed much before the following April. 

THE INFLORESCENCE AND THE FLOWERING-PERIOD. 

The growth-period of guayule is indeterminate and is largely a re- 
sponse to moisture conditions, within certain relativeh' wide limits (Chap- 
ter IV). Similarly, the formation of flower-buds occurs as a function of 
this growth and is not related to temperature or other seasonal conditions. 
Thus, if the growth is small in amount only that flower-bud which hap- 
pens to be ready to expand will be developed. If the amount is great a 
second or even third series of flower-buds may be developed and come into 
fruition, though it is seldom that more than two series mature in one year. 
When the summer rains commence the resting buds, with their frequently 
inclosed and partially developed flower-buds, soon begin to grow, and 
forthwith the first series of flowers is developed. 

According to my data for 1908 there was practically no growth at all 
till somewhat later than May 22. By June 9, in more favorable situations, 
as in arroyo beds, plants were found in full flower, and by about the mid- 
dle of the month flowering was well started on the ridges of the foot-slopes 
and in the hills. In certain unfavorable localities, e.g., on low ridges in 
the plains west of Cedros, the peduncles had attained, by July 22, only 
half their normal growth. The flowering of the hill plants continued for 
a month, seed ripening and new flowers coming on, when, by the middle 
of August, the vigorous flowering-period was entirely closed. By Sep- 
tember 9, up to which time there was more or less spasmodic flowering, 



60 Guayule. 

the period was at an end. This does not mean, however, that fresh flower- 
buds were not available and ready to develop, but that the water-supply 
was insufficient to support the heavy foliage and to enable the full devel- 
opment of the flowers as well. 

The end of the flowering season is shown as much by the abortion of 
the immature capitula as by any other behavior. This is but the extreme 
expression of a very general phenomenon, that of the unequal development 
of the inflorescence in adjoining situations. When water is abundant 
the inflorescence is widely spreading, the result of the development of 
the pedicels (fig. lo), while where the water-supply is meager, but not 
insufficient for the development of the flowers, the pedicels may remain 
very short and thus produce a crowded mass of capitula. This condition 
is usually met with in the field (plate 2), and between this and complete 
abortion of the flowers every degree of failure to flower is seen, the result 
of reduced water-supply. 

While the grand flowering-period falls normally in the summer, the 
exigencies of rainfall may so modify the rhythm of the plant that it will 
occur in, possibly, any month of the year. Under irrigation flowering 
starts in March, ^ and there is sustained a profusion of flowers through 
April (plate 14, fig. A) and May. It then dwindles, a second period of 
low maximum occurring in August, to be continued irregularly and with 
less perfectly developed flowers into November. In the field abundant 
flowers were observed in October in Durango (Hacienda de los Sombre- 
retillos) and in Sierra Ramirez, Zacatecas. Up to this time of the same 
year (1907) no flowers had been produced in the Sierra Mojada on the 
Hacienda Santa Inez, in Durango, where the guayule plants, forming an 
almost pure culture, were in a shriveled condition for lack of water. 

Under favorable conditions the development of the inflorescence takes 
about two weeks. The flowers emit a delightful fragrance which attracts 
many small insects. Among these visitors mosquitoes were observed, 
extracting the nectar from the ray-flowers. 

THE PRODUCTION OF SEED. 

Though the maximum number of seeds which may be produced by 
each capitulum is only 5, the total number yielded by a moderate-sized 
plant may amount to many thousands. The percentage of viable seed, 
however, runs small. In a field-plant with well-developed heads less than 
5 per cent of well-filled achenes were found. In other plants as high as 
2 5 per cent were found filled. In irrigated plants the percentage rises con- 
siderably higher, namely, to about 35 per cent. When the achenes are 
fully ripe the bracts become brown in color and fall away from the pedi- 
cels quite easily. The collection of seed (Chapter IX), which must be 
done by hand if at all, should begin to give the best results at the close of 
the first period of flowering. Properly done, the flowers are stripped from 
the peduncle, which need not be removed from the plant. The nature of 
the "seed" has already been discussed. 

* In 1909 flowering did not begin in these plants before the middle of April. 
Inquiry developed that they had not been irrigated freely, if at all, though of 
course the soil was much better supplied with moisture than that of the field. 



CHAPTER IV. 
REPRODUCTION. 

METHODS OF REPRODUCTION. 

It is the purpose of the present chapter to compare the two methods 
of reproduction, sexual and vegetative, with reference to final efficiency 
in reproducing the species. It need scarcely be said that, in speaking of 
sexual reproduction, we are using the term to indicate the origin of the 
seed. It will be at once accepted that accurate knowledge of the topic here 
to be considered is of vital importance in deriving estimates of the rate 
at which guayule fields may be expected to produce a crop of that plant. 

From what has been said in the foregoing chapter it will be seen 
that, taking different kinds of habitats into account, an average rate of 
reproduction will be maintained by means of the retono and seed methods 
combined. The relative efficiency of the two methods depends upon 
widely different considerations, and these, as we shall now see, have rela- 
tion to numbers of individuals, rate of growth, and the time of the year at 
which they begin this growth. 

RETONOS, NORMAL AND INDUCED. 

We may speak of two kinds of retonos, normal and induced. By 
normal we mean those which arise spontaneously upon the lateral, 
superficially placed, horizontal roots, remaining for some time attached 
to the plants from which they spring (plate 9). Induced retonos (plate 
15) will then be those which arise as the result of mutilation, that is, 
from roots, primary or of a higher order, after the plant has been cut 
away. This is done on a number of haciendas in the harvesting of the 
shrub, whereas the plants used to be, and by many still are, pulled up by 
the roots. This pulling results, of course, in breaking away many of the 
roots, but the chief portion of the tap-root is removed, as also are consid- 
erable lengths of the other roots. As we shall see, the difference in effect 
upon reproduction is merely quantitative, as in both cases retonos may 
arise, but in very different numbers. In order to test this with as great 
accuracy as possible, quadrats of 100 square meters were cleared of the 
guayule both by the cutting and pulling methods, and the results were 
noted. These, for the quadrats observed, afford accurate data, which 
must be understood as of indicatory value only. It may well be believed 
that different meteorological conditions would have modified the results 
ver}^ considerably. Thus, if the experiments had been started just at 
the beginning of the summer rainy season more hopeful results might 
have been had, but we shall see that cutting at this time is for other 
reasons an unfortunate practice, and the evil resulting would offset the 
value of the data thus obtained. It is well, therefore, for economic rea- 
sons, that the data collated shall be well within bounds. In addition 
to experimentally obtained data, others derived from observation are 
given, and have already been discussed in part in Chapter III. 

61 



62 



Guayule. 



General Observations. 



It is generally believed that, after a field has been harvested of its 
guayule, it will reproduce itself in a short period of years, the length of 
which is a matter of opinion. Estimates on this point vary from 5 to 
10 years. ' As, however, this difference in length of reproductive period, 
which we may call the period of rotation, involves so large an error in 
returns on investment, an effort to get at the facts is eminently justified. 
From the botanical point of view, the rate of reproduction and of growth 
of desert plants has been so little studied that data bearing on these 
questions are of great importance, especially as the eye of civilization 
is being turned on the desert as a field in which must be developed the 
natural resources peculiar to it. 

Normal Retoxos. 
The number of plants which arise as retonos within a given area is, 
with probably few exceptions, small. 

Table 20. — Comparative numbers of seedlings and retoTios in given areas. 



T ^^oKf ,. Number of small plants 
^°^^''*>- (below 8 oz.). 


Station. 


Quadrat. 


Seedlings. 


Retoiios. 


8 

8 

9 

9 

10 

II 

12 


2 

I 
I 
2 


36 
14 
86 

5 

90 

232 

200 


4 
2 

4 
I 
8 





These numbers are accurate as far as they go, but they do not tell 
what proportion of all the plants of the quadrats mentioned arose as 
retonos. In the vicinity of Station 2 plants of this sort could easily be 
found, and all but one of those in plate 9, fig. B, were obtained in a restricted 
area nearby, especially on the steeper slopes. But for all this, the total 
numbers of plants which have arisen as seedlings, taking all the areas 
into consideration, must far outnumber retono plants. On irrigated plants 
2 years old, some 150 in number, not a single retono was observed, a 
fact which may perhaps be correlated with the weaker development of 
shallow lateral roots in such plants. Only one instance (plate 46, fig. B) of 
a retono starting under irrigation has come to my notice. Numerous ad- 
ventitious buds were distributed on the mother-root, evidently having 
started after the plant was pollarded. This was done, not at the time of 
transplanting, but some time later, when it was discovered that the plant 
was not responding. The importance of normal retonos, therefore, is not 
to be seen in the numbers but in other qualities (Chapter III). 

' At the present writing we read in a recent number of the India Rubber World 
(March 1909), that a new crop of guayule may be expected in " a few years." We 
may suppose that heavily interested investors have obtained accurate information 
upon which they base their operations, but none, so far as we are aware, have been 
given publicity. 



Reproduction. 



63 



Normal retonos usually begin their growth with the oncoming of 
rain, especially in spring and early summer. In this regard they act 
merely as expressions of growth and have no special peculiarities. Start- 
ing as they do from the shallow-lying roots, they make an etiolated growth 
of a few centimeters before emerging from the soil. Their rate of growth 
depends upon the size of the root from which they spring and the num- 
bers arising at one point. If the root is slender growth is relatively slow, 
and subsequently depends on the rate of secondary growth of its distal 
portion; if large, the retono grows rapidly and may in a month or two 
attain a height of lo or 1 5 cm., a rate scarcely to be met with in the case of 
seedlings. A notion of the rate of growth may be had from the follow- 
ing table of measurements, based upon the specimens in plate 9, fig. B, 
the numbers referring to those similarly numbered in the figure. 

Table 21. — Size, age and weight of plants which arose as retonos 
(referring to plants in plate 9, fig. B.) 



Plant. 


No. of 

stems 

at base. 


Height of 
stem. 


Diameter of 
stems. 


Weight, fresh. 


Age. 


I . 

2-. 

3 • 

4 • 

5 ■ 

6 . 

7 • 

8 . 

9 ■ 

^lO . 


4 

I 
7 
5 
3 
3 
I 

4 
2 

4 
3 


cm. 
43 

28 

20 

16 

14 

20 

18 
14 
13 

10 
6 


mm. 

\ ^3 1 
21 

24 
I 16 J 
20 
4.5 to 8 
5 to 8 . 5 
5 to 8 
7.5 to 10 
173 
4 
3 
1.5 to 2 . 5 

2-5 


Ihs. OS. 
»2 8 

8 

3.5 
6 

2.875 
3-25 
3 
'0.56 


yrs. mos. 

8 to 9 

8 
4 
4 
3 
3 
4 
2 
2 

4 
2 to 3 



iDry weight i lb. s oz. 
2 Dry weight. 



' Induced by cutting away the plant, January 1908. 
^Grew m season of 1908. 



It is at once apparent that, as compared with the rate of growth of 
seedlings, that of rotonos is much more rapid. It takes at least 15 years 
to produce a plant of 2 pounds weight from the seed. Plant No. i , in the 
above table, made its weight in certainly not more than 9 years, possibly 
in 8. This is brought about by (i) the more numerous stems arising from 
the base and (2) the more rapid elongation of the stems, due to the ad- 
vantage had in the already established root-system. Table 20 affords 
comparative data as between seedlings and retonos. Incidental advan- 
tages accruing from this purely vegetative method of reproduction are (i) 
relative certainty of success because of the previous establishment of the 
parent plant , with relative independence of an initial good season in order 
to start, and (2) the rapidity with which the plants arrive at a condition 
to flower abundantly ; e.g., plant No. 1 1 , a few months old, produced fully 
100 seeds. These, in a desert especially, are no mean advantages. Thus, 
they would enable a single guayule plant to compete with such a plant 
as the lechuguilla, assuming that it had so fully occupied the ground that 



64 



Guayule. 



seeds could not get started, by maintaining a foothold till the dying off of 

lechuguilla plants, say as the result of flowering, allowed seedlings once 

more to take hold. 

Induced Retonos. 

In order to determine the number of retonos formed after pulling up 
(usually called "cortando") and after cutting away guayule plants, the 
following experiments were made: 

Experiment 11$. — Station 2, quadrat 4. Jan. 6, 1908. 250 plants, all 
under 40 cm. in height, were pulled up by hand, leaving in the ground only 
such roots as were broken off by chance. Feb. 18, no growth; Mar. 29, 
no growth; Apr. 28, 5 roots produced retonos ; July 28,9 clumps of shoots 
from as many roots started. Sept. 12, none additional. Apr. 3, 1909, 6 
additional roots had started. 

The following measurements were made of dried material collected 
on April 3, 1909 : 

Table 22. — Station 2, quadrat 4. Induced retonos. 



No. of 
stems on 
each root. 


Height of stem. 


Diameter of 
stem at base. 


No. of 
stems on 
each root. 


Height of stem. 


Diameter of 
stem at base. 




cm. 


mm. 




cm. 


mm. 


2 


13.5 


7, 8 


I 


6 


3 


2 


13 


3. 10 


5 


4 to 7 


3 to 4 


2 


15 


7. 9 


6 


3 to 4 . 5 


2 to 3 


I 


9 


10 


3 


4 to 8 


2 to 4 


I 


IO-5 


8 


3 


4 . 5 to 7 


I to 4 


6 


6 


3 to 4 


25 


I to 7 


I to 4 


I 


8 


5 


2 


9, 10 


4, 4-5 


10 


2 to 7 


2 to 4 









The average amount of growth in stem -length was 8 cm. ; in diameter 
4.4 mm. All of the new growths produced flowers, and were in normal 
condition when examined at the close of a long drought period. One of 
them is shown in plate 15, fig. A. 

Experiment 114. — Station 2, quadrat 3, Jan. 6, 1908. Of 338 plants, 
all but 88 small ones {i.e., 250 plants) were cut off with a "talacho" 
from I to 5 cm. below the surface of the ground. No growth till after Mar. 
29. Apr. 28, 1908, 40 clumps of new shoots well started, each clump of 
2 to 6 shoots . Stems 4 to 6 cm. long, with leaves of the same length. The 
severed roots died back about 2 cm. before the new shoots started. Depth 
of soil at which the shoots started, 2.4 cm. July 28, 59 clumps of new 
shoots. Sept. 12, none additional. Length of longest stems, 10 cm. On 
Apr. 3, 1909, 6 clumps were removed and measured, the data from which 
are given in table 23. 

Table 23. — Station 2, quadrat 3. Induced retonos. 



No. of stems 
in clump. 


Length. 


Diameter. 


No. of stems 
in clump. 


Length. 


Diameter. 


2 

12 
2 

Ave. 


cm. 
12 

7 
9 

8.3 


mm. 

5 
I to 4 

I, 4 

5 


4 
I 

I 

Ave. 


cm. 

6 

6 
10 

8.3 


mm. 
I to 5 

'3-5 
8 


5 



iProm a tap-root only 4.5 mm. in diameter. 



Reproduction. 



65 



Close by this quadrat a retono (plate 15, fig. B) was collected, which 
had sprung from a lateral root 5 mm. in diameter. The chief shoot had 
10 branches. Total height, 8.5 cm.; diameter at base, 5 mm. Number 
of inflorescences 8, producing 80 to 120 seeds. 

Station 2 , quadrats 5 and 6 . Apr. 3 , 1 909 . The following samples were 
taken at random, supplying the attached data for growth in 1908 : 

From a bro ken-off tap-root 7 mm. in diameter, 2 new shoots, 12.5 
and 13 cm. long by 6.5 and 7 mm. in diameter, respectively. 

From a broken-off tap-root 6 mm. in diameter, two new shoots 6.5 
and 3 cm. long by 3.5 and i mm. in diameter, respectively. 

From a lateral root, a clump of 5 stems, each 2 cm. long. 

Experiment no. — Station 3 (one quadrat). Dec. 31, 1907. 30 
plants, 30 to 60 cm. tall, cut off with a talacho. No growth observed on 
May I following, till which date there was no rain. July 16, 3 roots 
had started. A number of roots, including the 3 which had started, 
were taken up for examination and the data tabulated as follows: 

Table 24. — Station^ (exp. no). 



No. 


Order. 


Position. 


Died back. 


Diameter 

of root 
where cut. 


I 
2 

3 

5 
6 

7 
8 

9 
10 


Secondary 


Nearly horizontal .... 
Do 


cm. 
25 
13 

7-5 
13 
12 

7 


> Failed to start. 


mm. 

' IO-5 
8.0 

, 4.0 

5-5 

II .0 

. 17.0 
50 
4.0 

6.5 


Do 


Do 


Do 


Do 


Do 


Primary 


Vertical 


Do 


Do 


Secondary ' 


Horizontal 


20 


Do 


4<:° 


2.6; merely started 
13; started; shoot 

2 . 5 mm. long, 
ho. 5 


Do.. 


th 


....Do 


Horizontal 








1 



1 Arising from No. 6 at 2 cm. from the top, where cut. 
2 Started (in 1907 ?) shoot 7.5 cm. long. 

Experiment 121. — Station 4, quadrat i. 50 plants in alk These 
were cut away as in the other experiments, Jan. 14, 1908. May 6, no 
growth whatever apparent. A rain-gage was placed at this station on 
Jan. 14. May 6: rainfall registered to this date, 1.52 cm. 

Apr. 5, 1909, 6 cltMnps of retonos. This appearance of new growth 
followed on further rainfall, as evidenced by the rain-gage of Station 5, 
a short distance away. 

Experiment 125. — Station 5, quadrat i. Jan. 15, 1908. 275 plants 
cut off below level of ground with a talacho. May 6, 30 roots have started, 
sending up i to 5 shoots each, but smaller than those at Station 2. Between 
Jan. 15 and May 6, rainfall 353 mm. 

Apr. 5, 1909. 43 clumps of retonos. The increase in numbers was 
the result of the additional rainfall, as indicated by the rain-gage, which 
was still in position, though standing somewhat obliquely. Evidently 
some of the water had been lost, as the oil had disappeared. The amount 
remaining, 700 c.c, indicated a total precipitation of at least 850 mm. 
Length of new stems, 7 to 15 cm., with diameter of 2 to 9 mm. 

Experiment in. — Station 3. Dec. 31, 1907. 100 square meters. 
30 plants cut off at surface of ground. No new growths till after May i. 
July 15, one retono. Apr. 2, 1909, two retonos in all. 
5 



66 Guayule. 

The percentages of removed plants represented by new shoots in 
all the above experiments are as follows: 

Per cent, l Per cent. 

Experiment 115 (pulling) 6 Experiment 1 10 (cutting) 10 

Experiment iii (cutting) 6 } Experiment 121 (cutting) 12 

Experiment 114 (cutting) '23 j Experiment 125 (cutting) 15 

From the above data the following conclusions may be drawn: 

Retonos are formed much more easily from the stock left after cut- 
ting at or near the level of the ground. The probability that the plants 
removed will be represented by new growths after cutting is much greater 
when a portion of the stem at the top of the tap-root is left. This is due, 
of course, to the presence of numerous dormant buds. 

The promptness with which retonos start after cutting away the 
plants depends, in the absence of sufficient soil-moisture, upon the rain- 
fall. It is worthy of note (i) that these retonos may start slowly before 
the advent of rain, and (2) that the roots may die back at least 13 cm. 
before starting. Root No. i, experiment no, had died back 25 cm. dur- 
ing six and a half months, that is, at the rate of about 4 cm. per month, 
and it finally failed to start. It was a very dry period, and this long 
tenacity of life illustrates in a striking way the physiological resistance 
of these roots in desert conditions. 

While this degree of hardiness would serve very effectively to pre- 
serve the species under unfavorable circumstances, it is evident from our 
figures that the number of new plants produced is not as great as is gen- 
erally supposed. The best result obtained (exp. 114) indicates that under 
the conditions surrounding this experiment scarcely more than 25 per 
cent of the original stand may be expected. It is a matter for satisfac- 
tion, however, that even under the most drastic treatment a field of 
guayule mav be expected to reestablish itself in the course of time, since 
the new growths will in a short time be able to produce seed and these 
will contribute to the repopulation of the area. 

In April 1909, two areas were visited from which the guayule had 
been removed by pulling up the shrub. It appeared that only the larger 
plants had been removed, and that both places still contained the natural 
growth of smaller plants. The point of interest in this connection is that 
in one of the areas, the Lomerio de Zorrillos, it was very easy to find 
broken-off roots which had started to grow again, and retonos of various 
sizes up to 8 cm. were found. In the other area, in the Sierra de Ramirez, 
the ground was very hard and the peons found difficulty in pulling the 
plants out. Instead, they had twisted them off just above the surface, 
and from the butts remaining, with very few exceptions, new shoots had 
grown during the season of 1908, these measuring from 3 to 8 cm. in 
height. This parallels the behavior of plants cut off at some distance 
above the surface of the ground. 

Experiment 60. — Station 2, quadrat i. 25 square meters. Nov. 5, 
1907. 140 plants cut oft" at a height of 8 to 10 cm. above the sur- 
face of ground. 

' The total number was not determined in April 1909, but would doubtless 
have indicated a larger percentage. 



Reproduction. 67 

Jan. 6, igo8. Many buds 2 mm. long. 

Feb. 18. All but 5 plants budded. Longest leaves, 25 mm. 

Mar. 29. Little change. Longest leaves, 30 mm. 

July 28. 5 plants dead. Longest stems of new shoots, 7 cm. 

Sept. 12. 12 dead altogether. New shoots 10 to 15 cm. long. Plenty 

of flowers. Some plants have the appearance of witches' broom. 
Apr. 3, 1909. 13 dead. Maximum stem-growth, 20 cm.; minimum, 

3 to 5 cm. New shoots in several cases killed by drought (plate 

16, figs. A to C). 
Experiment 56. — Station i, quadrat 2 (25 square meters). All plants cut 

ofif as in experiment 60, Nov. 5, 1907. 
Jan. 3, 1908. No growth. 
May 29. New stems (upwards of 15 mm. long, 4 mm. diameter) 

on the majority of cut stems. 
Apr. 3, 1909. Maximum growth, 10 cm. stem-length. 
Experiment 126. — Station 5, quadrat 2. Jan. 15, 1908. All plants cut 

at 15 cm. above ground. 
May 6. Nearly all well budded. 
Apr. 5, 1909. New shoots 10 to 15 mm. long; flowered well in 1908. 

From the rainfall data it appears conclusive that the best time to cut 
guayule, with reference to reproduction by retonos, is just before and dur- 
ing the rainy season. As we shall see, however (Chapter V), this is the 
period of active growth, and the rate at which the accumulation of rub- 
ber takes place is such as to indicate that the practice of removing guay- 
ule at this time is not advisable. Therefore, other considerations aside 
(such as competition with other plants) , the removal of guayule even dur- 
ing the most trying seasons will not exterminate the plant, except on re- 
stricted areas which may be rehabilitated by spreading through seed. It is 
scarcely to be doubted that even in the quadrat of experiment 121a few 
retonos made their appearance after the last date of observation, which, 
unavoidably, was before the summer rains.' Furthermore, we are able to 
say from observation that the conditions at this station were more rigorous 
than at Station 2, where an earlier start was made by the retonos. 

The rate of growth of induced retonos will be seen to exceed the 
initial growth of seedlings. The stem-growth for the growing-season of 

1908, as shown by observations taken on the above experiments in April 

1909, was upwards of 15 cm., the average amount of growth falling some- 
where near to 8 cm. The retono in plate 9, fig. B, plant No. 10, made a 
stem-length of 10 cm. in about three months, and would probably have 
made more growth had it been allowed to remain. 

As between the pulling and cutting methods of gathering guayule, 
there can be no two ideas as to the relative effect upon the rate of repro- 
duction by means of retonos. In adjoining quadrats (experiments 114 
and 1 1 5) , in which it so happened that the same number of plants was 
removed, in the one by cutting and in the other by pulling, the clumps 
of retonos were as 59 to 15. This is explained by the fact that the roots 
left in the ground when the shrub is pulled up are not only fewer in num- 

* After this was written this quadrat was visited on April 5, 1909, and it is of 
interest to note that the behef expressed was substantiated. See experiment 121, 
above, on page 65. 



68 Guayule. 

ber but smaller than those left when the shrub is cut. The larger break 
off further in the ground and are therefore less favorably placed for starting 
afresh. The disadvantage of the cutting method in the eyes of those who 
are in pursuit of the greatest possible initial return is that less tonnage 
per acre is obtained, a loss, however, which would be made good many 
times in new plants if the roots were properly cut and allowed to remain. 

SEED. 

VIABILITY. 

The seeds of guayule appear to have a fairly long period of vitality, 
a conclusion, however, which is inferential and has not been demonstrated 
by direct experiment. The view is based on the following experiment 
(exp. 78): On November 23, 1907, a lot of trays (such as are shown in 
plate 45) were filled with paper tubes of i square inch transverse section. 
The trays were then filled with soil made up of half and half garden soil 
and old dry manure from a horse corral. In the top of each tube were 
sown 20 to 30 seeds. The trays were watered abundantly by subirriga- 
tion, it being the purpose to try the method of using the trays with paper 
tubes for wholesale germination. So far as this was concerned, the experi- 
ment was a failure, but it served to contribute to our knowledge of seed 
vitality. The very dry season made it very difficult to keep the surface 
soil moist, and as a result of alternate drying and wetting the upper part 
of the soil became caked and there was considerable efflorescence of salts. 
The soil below became soggy and sour, and fungi permeated the soil and 
the paper of the tubes. Very few seeds germinated, not more than one 
or two in each tray, partly, as was later determined, because of the char- 
acter of the soil, and partly because of the prevailing low temperatures. 
The trays lay thus, occasionally wet by showers, till the following July, 
when a large number of seeds started to germinate. In one tray 138 tubes 
had seedlings, from one to eight in each. By July 25 the seedlings had 
developed two foliage leaves, and by August 28 a stem-growth of 5 cm. 
was not exceptional, with leaves 5 cm. long. Some plants had at this 
date as many as seven foliage leaves. Thus it will be seen that the seeds 
which germinated did so after six months' exposure to conditions about 
as bad as could be imagined, being alternately wet and dry, in a sour 
soil, and open to the attacks of fungi. The germination in trays favorably 
placed with respect to shade was upwards of 13 per cent of viable seed, 
as nearly as may be calculated.^ It may therefore be concluded that the 
seed of guayule, being neither very short-lived nor very sensitive to unto- 
ward conditions, is, from a biological point of view, quite efficient for the 
preservation of the species.^ 

It is, in the nature of the case, well-nigh impossible to determine 
the percentage of germination of nature-strewn seed, but one successful 
experiment affords us exact data (experiment 192). On May 30, after a 

* Critical germination tests to determine the viability of seed have been 
made by Kirkwood (191 oa) , who finds the germinations to scarcely exceed 1 4 per cent , 
and that after eight months there is a marked drop in viability. 

^ Ready germination from seed collected during the summer of 1908 was 
obtained in July 1909, at Auburn, Ala. 



PLATE 16 












^^^'\:^ 




A— C. New growths aher pollarding. A, February 18, 1908; 
B. March 29. 1908; C. April 5. 1909. 
D. Seedlings in limestone soil; E, in "garden" soil. 








A. Minimum, average, and maximum seedlings. (Station 2, quadrat 4.) 

B. Irrigated plant, two years old, from a stock. April 1909. Cedros. 



Reproduction. 69 

rain of 16.8 mm. during the preceding night, 4 ounces of seed (including 
chaff) were sown at Station 7, in 5 rows, each a meter long. The ground 
was previously cleared of all plants and thus loosened, and, the seed 
having been left uncovered, the seedlings were exposed to full insolation. 
On September 9 following, 119 seedlings were counted. ' These compared 
favorably in appearance and size with other seedlings found growing 
spontaneously in the surrounding area. The seed was sown more thickly 
than would occur in nature, and the number of seedHngs was also much 
greater, and far too great for their normal development.^ 

Comparison of these results with those obtained by observation of 
germination in irrigated ground affords considerable interest. About 
150 plants, placed in a small patch of ground by Mr. C. T. Andrews in 
the spring of 1907, flowered freely during that and the following year. 
A very large number of seeds must have been disseminated, notwith- 
standing a good deal had been gathered, of which fully 30 per cent were 
viable. During the summer of 1908, at the time (June) when seed was 
germinating in the surrounding region under natural conditions, some 
seedlings were observed. About 50 were counted, but in the whole 
area (o.i acre) there could hardly have been more than a few hundred 
at the outside. Nor did they grow as well as field seedlings, perhaps 
because of the rapid drying of the superficial layers of soil. The percent- 
age of germination here must therefore have been exceedingly small, 
and much less than that which occurred in experiment 192 above de- 
scribed, and also than that which takes place in nature, if we may judge 
by the numbers of seedlings actually found in the field in the summer 
of 1908. The following observations are pertinent here: 

(i) Station 3. June 1908. In areas of i square meter, representative 
counts of 8 and 14. April 1909, 23 living seedlings of 1908 were 
found on the whole quadrat (100 square meters). 

(2) The region about Stations 7 and 8. On June 24 a large number of 

seedlings was seen. 

(3) Station 2, quadrats 5 and 6. Sept. 12. Four seedlings 10 cm. apart. 

Nearby 6 seedlings 10 cm. apart. Several counts showed about 
20 plantlets per square meter. None on previous visit to this 
station, July 28. 

(4) In I square foot on the same area, 6 well-grown seedlings. Sept. 

12, 1908. 

(5) In a wire-fenced quadrat which was cleared of all plants (other 

than guayule) by Mr. C. T. Andrews early in 1907, 5 miles north 
of Cedros in an open plain, leaving one tall guayule plant in the 
middle, no seedlings appeared till after June. In September 29 
seedlings were found within 6 feet of the plant, chiefly in one 
direction. One mariola seedling was found. 

(6) Station d), quadrat 1 (100 square meters). 24 seedlings, Sept. 1908. 

(7) In 4 square feet, on a loma north of Cedros, near Station 8, Aug. 8, 

31 seedlings, all of 1908 except one of 1907. This number in- 
cluded one of Parthenium hysterophorus. 

*The largest of these had a stem (epicotyl) 1 cm. long, with leaves 4.7 cm. 
long by 1.5 cm. broad. 

' In April 1909 it was found that all the seedlings had been destroyed by goats. 



70 



Giiayule. 



(8) Near this place 22 seedlings were collected from two areas, each of 

12 square inches; i of 1906, 8 of 1907, and 13 of 1908. 

(9) Station 2, quadrat 4. April 1909. 281 living seedlings, all of which 

germinated during the growing-season of 1908, were collected on 
100 square meters (plate 17, fig. A). 

(10) Endlich reports finding "as many as 50 young plants around full- 

grown trees" (335, 1905, Eng. tr.). Such a large number is not 
common, but it is not unusual to find 25 seedlings with two foliage 
leaves about the base of a single plant. 

From such observations it is clear that in particular areas one may 
find by chance many more seedlings than could by any fortune develop 
into mature plants. Other areas, however, are quite bare of them. Again, 
many seedlings which get started die in the course of time, and there can 
be no doubt that the percentage of deaths is great. Counting seedlings, 
therefore, is not a dependable method of determining the rate of repopu- 
lation. For this purpose it is necessary to make a census of sample quad- 
rats, making as careful estimates as possible of the sizes and ages of the 
plants. The data in tables 4 to 13 afford such a census. They are summa- 
rized in table 25 and are further displayed graphically in fig. 12. 

Table 25. — Classification of giiayule plants from seed according to weight on various 

quadrats indicated. 



Quadrat. 



Table 4 
6 

7 
8 

^9 
10 
II 
12 
13 



4 lbs. or 
more. 



3 lbs. or 
more. 



2 lbs. or 
more. 



13 
13 
12 

o( 
5 



I lb. or 
more. 



10 

15 
40 
20 

o(?) 

4 
28 

23 
45 



i lb. or 
more. 



4 
9 

25o(?) 

4 

59 

23 

53 



Less than 
ilb. 



585 
26 
10 
86 

755 

6 

90 

232 

166 



1 It is to be recalled that the larger plants had previously been removed from this quadrat. The 
estimate marked doubtful is based on the figures of adjoining quadrats, and can only be approximate. 

It is clear that the ratios between small and large plants, as shown 
in table 25, indicate very different degrees of efficacy in reproduction 
commencing from the seed. This method is the best available in the 
absence of actual counts of seedlings year by year, obviously not practi- 
cable. A few such counts, for future comparison, are given in table 26. 

Table 26. 





Seedlings 
of 1908. 


When counted. 


Station 8, quadrat i 


24 

23 
281 


1908 

Apr. 2, 1909 
Apr. 3, 1909 


Station 3 , quadrat r 


Station 2 , quadrat 4 





These few data, the difficulty of obtaining which, on account of vari- 
ous circumstances, was very great, have only suggestive significance. It is 



Reproduction. 



71 



obvious that in the last station reproduction by seedlings is relatively very 
good, especially as the counts were made at the close of a long drought. 
A condition such as this might, in the light of table 25, be expected to 
lead to a good stand of guayule. From a consideration of the curves 
based upon table 25, some further points of interest are discovered. There 
is a large falling off in numbers of plants between the average weight 
of about 4 and 12 ounces. This, as seen in the curves on pages 87 and 88, 
is the period, approximately, of maximum rate of growth, viz, between 
8 to 10 and 13 to 15 years of age, during which time there is a loss of 
total weight of about one-fourth to one-third, as nearly as we may calcu- 
late. From the nature of the conditions, many of which are undetermi- 
nable, such calculations can be only loosely approximate, but it can hardly 
be doubted that, if the rate of reproduction by seed from plants, say 
from 6'^to 8 ounces in weight, can be depended upon quantitatively as 



? 400 




2-3 

POUNDS 



Fig. 12. — The relative numbers of various-sized plants on different quadrats. The numbers 
at the ends of curves refer to the tables corresponding. 

indicated in the table under consideration, it is an economic loss to allow 
plants larger than these to remain. From this point of view alone it 
may not pay to allow the plants to remain after the age indicated by 
the weight of 6 to 8 ounces has been attained, as the numbers which die off 
are great enough to cause a considerable falling off of total weight. 

The data show also that the initial monetary return from a harvest- 
ing of guayule may be as great or greater from a stand of a few large 
individuals, but the areas with large numbers of smaller plants give 
promise of future returns. 

An important desideratum is to determine how to improve these 
conditions. Here, let us say, is a good field of guayule, as regards first 
returns. The bulk of the weight is in large plants, and the small ones 
are too few for a ready reseeding of the area after depletion. It is hardly 
too much to say that vast areas are in this condition. What may be 



72 Guayule. 

done to increase their productivity is still a question for experimental 
determination, but seeding in favorable years by means of seed from 
densely grown areas would be distinctly beneficial. The importance of 
seed is so great that in the harvesting of shrub the practice of leaving 
large plants for the purpose of producing seed should in all circumstances 
be initiated. As a practical question of economics, the difficulties of 
time and distance in the desert are so great, not to mention those arising 
in connection with climatic irregularities, that any attempts to better 
conditions over wide areas are fraught with expense which may not be 
considered as warranted by those interested. 

COMPARATIVE ABILITY TO GERMINATE IN THE FIELD. 

The ability to germinate promptly, to attain a condition of physio- 
logical resistance, is of prime importance to desert plants, and very much 
more important to them than to plants which are more favorably placed 
with reference to water-supply (Ganong, 1907; Lloyd, 1909a). So far as 
the question of germination is concerned the evidence is not forthcoming 
that desert plants exhibit more indifference to initial water-supply than 
others (Livingston, 1906). For the rest, as for further elucidation of 
this problem, much comparative study is necessary. There seems to be 
little doubt, however, that the rate at which physiological resistance is 
acquired and the amount of this resistance are very different in different 
plants. For example, the seedlings of many succulents soon acquire 
the characters of the parents, the cacti (Ganong, 1898) being notable 
examples of this. This must be of no small weight as a factor in enabling 
young plants to withstand the rigors of drought, though this very cir- 
cumstance in the cacti opens them to the attacks of animals (MacDougal, 
1 910), so that millions of seedlings are eaten, affording both food and 
water to desert animals. 

As has been shown, and as will be further developed in the following 
chapter, the guayule seedling offers no exception to the rule that desert 
plants need an abundance of water during the period of germination. 
Observation in the field indicates further that marked readiness in ger- 
mination is not in any way indicative of adaptation to desert conditions. 
A field test of the germinating ability of guayule in comparison with 
that of alfilaria (Station 7, May 30, 1908, exp. 139) showed that about 
3 per cent of the seed of the latter germinated, while about 0.2 or 0.3 per 
cent of guayule succeeded in getting a foothold in the same place under 
the same conditions. These figures are probably too low for both plants, 
inasmuch as ants were observed carrying off seed on each occasion that 
the station was Adsited. This test, however, may indicate the direction 
in which research may contribute toward the explanation of the success 
which the alfilaria has had in invading desert territory. 

A further observation was made at Camacho, on the Mexican Central 
Railway, on the Hacienda de Cedros, a point for the shipment of guayule, 
where a stack-ground had been kept supplied with shrub from the neigh- 
boring region. It is customary to bring in the shrub in loose bundles on 
the backs of burros or in carts of various sizes and kinds to these ship- 
ping-points, there to be made up into bales for handling on the railroad. 





Seedlings growing in different soils: A, May 25. 1908; B, April 13, 1908. 



LUOYO 



PLATE 19 




LLOYD 





A. (1) Root-cutting; (2 to 4) Sectional root-stem cuttings (Exp. 146). 

B. Seedlings grown in different soils, August 1908. 



Reproduction. 73 

Countless numbers of seeds are therefore strewn upon the ground, and 
indeed the new plants of guayule which spring up on these stack-grounds 
sometimes afford valuable data on the rate of growth, although decep- 
tive notions as to the numbers of plants which may be expected are some- 
times acquired. The conditions of a stack-ground are, at Camacho at 
least, a rather severe test, as it lies out in the open, dry plain, exposed to 
full sunlight. At the same time, the surface of the soil is mulched by the 
debris of broken-off guayule twigs, and thus the conditions are amelior- 
ated. Shipments, which had been made for a year or longer, ceased in 
the fall of 1907, and the spot was under occasional observation for some 
time before and from that time on, till the following September. Al- 
though it is known that much guayule was brought in in flowering con- 
dition and that seed must have been dropped in large quantities, the 
conditions for germination, especially the meager rainfall, were not favor- 
able for guayule, though the seeds of the plants in the following list 
were found in all conditions of development in June and July of 1908: ^ 

Helianthus sp. 15 to 18 inches tall and many in flower. 
Amaranthus, 2 species. Plants 3 inches tall. 
Cassia ("coco"). Many mature plants in flower. 
• Prosopis seedlings with the plumule well developed. 

Euphorbia of 2 species. Mats 10 inches in diameter. 
Solanum sp. 
A cucurbitaceous vine. 

Cheno podium, a species with broad deltoid leaves. 
Grasses of 4 species. 
Spheralcea, mature plants. 

In addition to these seedlings, the roots of Prosopis and Covillea, 
which had been cut off in preparing the ground for stacking, had sent up 
shoots from 10 to 20 inches in length. That not a single guayule plant 
sprang up is at first surprising, not to say disconcerting, but in the light 
of experimental evidence it becomes clear that the guayule germinates 
only under highly favorable conditions. For some time it has a low de- 
gree of resistance, and is in point of fact of distinctly mesophytic charac- 
ter. It is only when due regard to this is had that the maximum rate of 
germination may be expected under cultural conditions. 

H.4BITATS OF SEEDLINGS. 

The particular preference of the guayule for certain germination 
habitats is of importance in its bearing on the effect of clearing land of 
other plants. It has been repeatedly observed by the investigators at 
the Desert Botanical Laboratory, and by myself in Zacatecas, that there 
are usually to be found many more plants of smaller size growing in the 
partial shade of shrubs than elsewhere, and it is to the protective effect 
of this shade that the many curious juxtapositions of perenifial plants 
may be referred. An example of this is the frequently seen saguaro 
(Carnegeia gigantea) , standing in a position indicating that it germinated 
in the shade of a palo verde {Parkinsonia microphylla) or some other 
shrubby species. 

As regards the guayule, Endlich (1905) speaks of "the large numbers 
of young plants sometimes found surrounding the older trees * * * in the 

• Every annual had disappeared by April 1909. 



74 Guayule. 

territory around Jimulco, for instance, as many as 50 young plants have 
been found around full-grown trees." But, on the other hand, speaking 
of the occurrence of young plants in supposedly very unfavorable spots, 
Endlich explains this by saying that "it is likely that they have been 
developed from such seeds as were either stamped into the ground by 
goats (as these animals are the ones which commonly graze in the guay- 
ule territories), or had been dropped by these animals and thus found 
favorable conditions of development in the animal excrements. It 
would, in fact, be difficult to find any other explanation for the enormous 
growth of the guayule plant in small, isolated places (having usually 
the size of the resting-places of the herds of goats) ^'' * * ." 

As to the supposedly favorable conditions afforded by animal excre- 
ment, it may well be doubted that these are more so than the soil itself 
affords. Experiments have shown that soil at all rich in humus derived 
from manure is distinctly unfavorable for healthy germination. Even 
"garden" soil at Cedros, with no addition of manure, is less favorable 
than the unaltered lime-charged soil of the normal guayule habitat (plate 
16, figures D and E). Even after thorough leaching from exposure the 
possible advantages are hardly important, and, at all events, in such 
situations the seeds and seedlings have no advantage of shade, as the 
herding-spots of goats are usually bare of vegetation. Nor can the 
stamping into the soil by these animals have any value, as the seeds ger- 
minate well only with very shallow soil covering, as much as 2 mm. 
depth being enough to show a marked decrease in germination.' It would 
seem, therefore, that if Endlich's observations are correct as to the occur- 
rence of guayule seedlings in such situations, it is safe to infer that the 
rainfall conditions are, on occasion, such as to make ready germination 
and early growth possible for a good percentage of seeds even in open 
bare spots where no advantage of shade is offered. My own observations, 
at any rate, sustain this view. Experiment 139 (see p. 72) is a case in 
point, and the results were supported by general observation during the 
summer of 1908, when there was a fairly generous if not a maximum field- 
germination. The net result of this season is indicated by the numbers 
of seedlings observed in April 1909 (see p. 70). These are known to 
have germinated at or during the growing-season of 1908, and had suc- 
cessfully sustained prolonged drought till the time of observation. At 
no other point was there seen a better crop of seedlings at "the age of 
these, and they germinated without the least protection, as the quadrat 
had been completeh^ denuded." 

Nevertheless, when seedlings are observed in the field at other than 
favorable seasons, it is frequently noticed that the larger numbers are 
in the protective shade of other plants ; but this is not peculiar to the 
guayule alone. The explanation, we believe, is not that the guayule 
seedling is ombrophile, but that the eliminating effect of the drought 
period subsequent to a period of germination is more drastic elsewhere 

* Kirkwood, 19 10. 

^ By contrast, it should be said that at Station i only very few seedlings 
were found on a large area denuded of all plants save small guayule. As goats 
had been pastured here, however, it is impossible to draw any conclusions. 



Reproduction. 75 

than in the shade. Thus, in February 1908, small seedlings with i to 5 
foliage leaves could be found beneath the shade of an occasional larger 
guayule plant, but in a precarious condition, some dead, others moribund, 
and plainly the survivors of the crop of seedlings of late in 1907, the chief 
part of which had succumbed to the very severe conditions already noted 
as having prevailed at Cedros at that time. As bearing upon this ques- 
tion, we may note the meager occurrence of Opuntia leptocaulis in south- 
ern Arizona, where it is scarcely to be found except protected by some 
plant, while it grows in the open in great abundance in Zacatecas. It 
appears evident that in Arizona the conditions for its persistence, except 
when it is more or less protected by other plants, are too severe. No such 
relation has been observed in Zacatecas, and it would seem that the cli- 
matic conditions there are distinctly more favorable for this plant. 

It would therefore appear safe, if desirable, to clear guayule fields 
of the major part of other vegetation. An occasional year may be ex- 
pected when the rate of germination will go far toward producing a good 
stand of young plants. Those already growing will offer protection to 
the younger brood, and the larger area available for guayule plants will 
in part compensate for the loss of shade given by other vegetation. It 
would not be advisable, however, to remove the occasional palma saman- 
doca {Samuella carnerosa) , which produces fiber, or the large barrel cacti 
(" bisnaga burra " and " bisnaga colorada ") , as they are heavy plants and 
neither spread with appreciable rapidity nor occupy more than a negli- 
gible fraction of the ground (plate i, fig. A). This principle of practice 
is, however, in the nature of a compromise, and rests upon an estimated 
balance of circumstances. A more correct estimate of probabilities could 
be based only upon longer observation under experimental conditions. 

RATE OF REPRODUCTION AND OF GROWTH. 
RATE OF GROWTH DURING GERMINATION. 

This period may be divided into a period of tissue expansion and 
one of induration. At the close of expansion, which begins in about a 
week's time after sufficient rain, and occupies a second week, the seedling 
is tender, the hypocotyl white and translucent, and the cotyledons green 
(fig. 8). The cuticle then thickens, and red color is developed in the 
epidermis of the hypocotyl and under surface of the cotyledons, while the 
latter become darker green and more indurated. This occupies a third 
week, when, if no untoward circumstance interferes, the first foliage 
leaves develop. Even under the best of conditions this period of three 
weeks will scarcely be shortened. 

The further seedling development is a direct function, other things 
being equal, of the rainfall, the maximum potentiality, it may safely be 
said, never being exerted by field plants. This apparently extremest 
limit of growth for a seedling was reached by one of two particular indi- 
viduals under cultivation, and constantly suppHed with an abundance 
of water. The height of this plant when the rhythm-limit was reached, 
as indicated by cessation of growth, was 25 cm., and it had a spread of 
22 cm. It was a fully-developed specimen, in which each branch reached 



76 



Guayule. 



its proportionate size. It flowered freely and produced fully 2000 seeds 
(exp. 139a; plates 18 to 20). The time occupied in its growth was about 
four months. 

We may now offer data (table 27) derived by field observation during 
the growing season of 1908, which was a favorable year, though not per- 
haps exceptionally so. The rate of growth during germination is indi- 
cated by the measurements of seedlings from Station 3, collected July 
15, 1908. They were two to three Aveeks old. 

Table 27. — Rate of growth during period of germination. 



Hypocotyl. 


Cotyledon. 


Length. 


Diameter. 


Length. 


Breadth. 


mm. 


mm. 


mm. 


mm. 


6.5 


0.75 


3-5 


3 


6.5 


0-5 


2.5 


2.5 


10. 


I .0 


4.0 


3-5 


II .0 


I .0 


4-5 


4.0 



Table 28 contains data based upon the individual examination of 1 1 2 
seedlings collected in the field on the dates mentioned. The measure- 
ments are exclusive of the hypocotyl, which measures about 10 mm. on 
the average. 

Table 28. — Ainount of growth of seedlings in tJie first season of growth. 



Collected. 


No. of 
seed- 
lings. 


"iSwv^el^'- Length of stem. 


Locality. 


Notes. 


Max. 


Min. 


Ave. 


Max. 


Min. 


Ave. 




Feb., 1908 

June 2, 1908 

Aug. 8, 1908 

Sept. 8, 1908 
Sept. 12, 1908 
Sept. 12, 1908 
Sept. 12. 1908 


II 

I 

28 

24 

6 

1 1 

31 


mm.. 

17 

33 

6S 

75 
90 

75 


mm. 

7 
6.5 

70 

6 

10 
47 
SO 
30 


10 


mm. 

2 


mm. 


30 



I 
5 
7 
3 


mm. 

i± 

i± 

5-7 
9.6 
24 
9 


Loma north 
of Cedros. 

Station 2 

Loma north 
of Cedros. 

Sta. 8, quad- 
rat I 

Station 2 

Station 2 

Bare quadrat 
in plain be- 
tween Ce- 
dros and 
Sta. 2. 


Germinated Nov. (?), 
1907. 2 to 5 foliage 
leaves. Cotyledons 
long gone. Plants of 
very slow growth. 

An exceptional and 
very large seedling 
for this date. Inflo- 
rescence 2.S cm. long. 

Cotyledons still attach- 
ed. I to 7 foliage 
leaves. 

Good healthy speci- 
mens. 
Do. 

2 in flower. Max. stem 

diam. ,3 mm. 
Good healthy plants. 


20 

35 
62 
73 
45 


2 

12 
17 
50 
20 




14.8 













The seedlings in table 28 were not selected, but were, in each 
case, all the seedlings found in a given area. Taking those collected in 
September — w^hich, judging by the behavior of guayule plants in gen- 
eral, was near the close of the growing-season — we have an average rate 



Reproduction. 



77 



of growth of about 14.4 mm. in stem-length (epicotyl) , aside from the small 
secondary branches. With few exceptions, the seedlings of a month pre- 
vious (August 8) were very small, as indicated in the table, but neverthe- 
less the size attained by them, judging from experience in their culture, 
must have been the result of at least six weeks' growth. 

This was not the close of the growing-season, but I was fortunately 
able to complement the above data by measurements of seedlings, already 
mentioned in other connections, which had passed completely through 
the growing-season of 1908 and been collected* in April 1909, in a state 
of dormancy. The measurements of 311 seedlings were made by caliper. 

Tables 29 and 30 give the data for two quadrats; a third, having 281 
seedlings, 4 of which are seen in plate 17, fig. A, is not given in detail. 

Combining the averages obtained from tables 29 and 30 with the data 
for Station 3, quadrat 4, obtained at the same time as those of Station 2, 
quadrat 7, we obtain table 31. 

Table 29. — Growth of seedlings which germinated abotit June i, 1908, and examined 
April 2, 1909. Station 3. All within 100 square meters. 





Length of main 






No. 


stem above 
hypocotyl. (Hy- 
pocotyl 10 mm.) 


Diameter at 
base. 


Remarks. 




mm. 


mm. 




I 


35 


3 -5 


Flowered, flower bitten oflf; branched. 


2 


19 


3 -2 


Branches i to 3 mm. 


3 


9 


2 .0 


Unbranched. 


4 


IO-5 


2 . 2 


Lateral buds just started. 


5 


I r 


2 .0 


Unbranched. 


6 


II 


2 .0 


Do. 


7 


5 


2.8 


Two buds at base of hypocotyl ; otherwise 
unbranched. 


8 


8 


2 .0 


Slender branch 5 mm. long at base of hypo- 
cotyl; otherwise unbranched. 


9 


6 


2-5 


Unbranched. 


10 


4 


2 .0 


Do. 


II 


5-2 


2 .0 


Do. 


12 


5-2 


2 .0 


Do. 


13 


5 -o 


2 .0 


Do. 


14 


4.2 


2 .0 


Slightly damaged ; unbranched. 


17 \ 


4 


1.8 


Unbranched. 


30 


1-7 


Do. 


18 J 








19 


4.0 


I . 2 


Do. 


20 


30 


I . 2 


Do. 


21 


2 -5 


I .0 


Do. 


22 


17.0 


0.8 


Unbranched; etiolated. 


23 
Ave. 


8.0 
8.1 


I .0 


Unbranched; slightly etiolated. 


1.8 



23 seedlings; average length of main stem, excluding Nos. 22 and 23, which are 
not normal, 7.65 mm.; average diameter of main stem at base, 2 mm. 

N.B. — The exact age of the above seedlings does not exceed 10 months. Of this 
period, 6^ months were without rain, beginning with the middle of September. All 
the seedlings were alive at the time of collection. 

' In company with Mr. G. E. Pell, of New York. 



78 



Gnayule. 



Table 30. — Growth of seedlings {all unbranched) which germifiated about June i. 
1908; collected April 3, 1909. Station 2, quadrat 7, 100 square meters. 



No. 


Length of main 

stem exclusive 

of hypocotyl 

(about 10 mm.). 


Diameter at 
base. 




mm. 


mm. 


I 


4.5 


2 ,0 


2 


5 


2 .0 


3 


5.0 


2 .0 


4 


6.0 


2 .0 


5 
6 


3-5 
6.0 


' -5 
1-3 


7 


50 


I .0 


Ave. 


S-O 


1-7 



Table 31. 



No. of seedlings 
in quadrat. 


Length of stem. 


Diameter of stem. 


Max. 


Min. 


Ave. 


Max. 


Min. 


Ave. 


23 

7 
281 

31J 
Ave. 


mm. 

35 
6 

55 
32 


mm. 

2-5 

3-5 
1.5 

2-5 


mm. 
8.1 

5-0 
12.6 

8.5 


mm. 

3-5 
2 .0 
6.0 

3-8 


mm. 
0.8 
I .0 
0.8 

0.9 


mm. 
1.8 

1-7 
2.8 


2.18 



It will be seen that the average maximum amount of growth for the 
whole of the growing-season of 1908, as indicated by the data obtained in 
April 1909, is 8.5 mm., stem-length. This, however (as shown by table 
28), is less than the amount determined by the measurement of seedlings, 
germinated in 1908 but collected on September 8 to 12 of that year, 
namely, 14.8 mm. The difference in favor of the earlier collections may 
perhaps be explained by the fact that care was not taken to take every 
seedling in a given area. To do this requires a minute search, which was 
given only in April 1909. It is not improbable also that other seeds 
germinated later in the season, though this is not likely. It is therefore 
safer to conclude that the average amount of growth in length of the 
epicotyledonary stem for the season of 1908, taking all seedlings into con- 
sideration, is not more than i cm. If we should consider only those w^iich 
germinated at one time, at the beginning of the growing-season, this 
amount would probably turn out to be somewhat greater. Under the 
conditions for the period in question the maximum amount of growth 
was 5.5 cm.; the minimum, 1.5 mm. Seedlings of these dimensions, and 
two illustrating the average growth of 281 seedlings (Station 2, quadrat 
4) , are reproduced in plate 1 7 , fig. A. Measurement of the main shoot alone 
throws out of account the growth of branches, so that a fvillcr conception 
of the amount of development possible for a seedling under natural con- 
ditions may be had only by seeing the plants themselves. 



Reproduction. 79 

RATE OF GROWTH IN MATURER PLANTS BICYOND THE 
SEEDLING STAGE. 

In general forestry practice the use of formulae is directed toward 
estimating the amount of lumber in the trunk. The deduction of these 
formulae is easier in the case of coniferous trees because of the continuous 
growth of the chief shoot. Special problems demand formula' based 
upon other data than the rate of growth of wood, e.g., in the business of 
producing cork from Quercus suher. When forestry practice is directed 
toward the culture of camphor trees, for example, in which the whole 
bulk of the plant is to be used, the desideratum will be to determine the 
rate of increase of weight. This is the case with guayule, since the whole 
of the plant is used in the process of extraction of crude rubber. But 
the rate of increase in weight can not be determined without introducing 
the time element, so that we must first determine the rate of stem elonga- 
tion in order to arrive at a general average of growth. But plants of the 
same age are not invariably, or even quite usually, of the same weight, 
since the relation of a plant to its environment results in more or in less 
bushiness, in partial death and consequent loss of branches, in unusually 
slow or rapid growth, or in total loss of plant by death. In estimating 
the weight of shrub per unit of area for some future time it is evident that 
all these factors are disturbing elements, the values of which may not be 
easily determined. About the best we can do, therefore, is (i) to determine 
the average rate of growth in length of stem, and (2) to determine the rate 
of increase in weight for critical periods. The data indicate that there is a 
period of relatively highest growth-rate, expressed in stem length or height, 
and a period of relatively greatest increase in total weight of the plant. 

RATE OF GROWTH IN TERMS OF STEM-LENGTH. 

It has already been shown that the first season's growth results in 
an average stem-length approximating i cm. A stem of this size has no 
branches. During the second season's growth the stem may simply 
lengthen, or it may also produce a number of short branches. This it 
is more certain to do if the chief shoot produces an inflorescence. It 
may otherwise merely elongate strictly for a number of years, resulting 
in a very slow increase in weight, since the weight is affected chiefly by 
the number of branches. At best the total weight assumed by a plant 
in the first 7 to 10 years is small, seldom exceeding a few ounces. 

RATE OF GROWTH IN EARLIER YEARS AFTER GERMINATION. 

To determine precisely the age of a given seedling is more dithcult 
than would seem at first glance if it has been exposed to the weather for 
more than a year. Furthermore, the rate of growth in many individuals 
is so slow that the marks become well-nigh efTaced. if not quite so. In 
obtaining the following measurements, only plants which showed the 
markings plainly enough to be seen clearly have been used. This has 
very naturally thrown out those of very slow growth, in which the diffi- 
culties are greatest, and thus the resulting average datum is probably 
too great. By way of orientation two extreme cases may be cited. One 
is a seedling of two seasons' growth, which germinated in 1907, making 



80 



Gaayide. 



in that year 3 cm. and in the following 3-ear 11 cm., a total of 14 cm. in 
the two years. This is the largest field plant for its age that I have seen. 
In contrast is cited a seedling of slow growth, fully 7 years of age, entirely 
without branches, and only 6 cm. in height. The average rate of growth 
falls between these extremes, but nearer the lower. For the sake of brev- 
ity, as it would serve no useful purpose to introduce large tables of fig- 
ures, the summaries of measurements alone are given. 

The average rate of growth of 30 seedlings from 2 to 5 years old during 
particular years is as follows: 

Table 32. 



Age. 


1908. 


1907. 


1906. 


1905. 


1904. 


Average amount of growth. 




mm. 


mm. 


mm. 


mm. 


mm. 


In first year, 17 mm. 


2 


15 


16 








In second year, 20 mm. 


3 
4 


45 
SI 


22 
30 


20 
24 


15 




In third year, 37 mm. 
In fourth year, 51 mm. 

31 mm. 



The average amount of growth in seven seedlings for the last three 
years, 1906-1908, is 26 mm. 

Some ten seedlings for each of the localities mentioned below were 
measured, giving average amount of growth for two to four years, as 
follows : 



Sierra Candelaria 22 

Station 4 (Sierra Guadaloupe) . 24 

Station 5 (Sierra Guadaloupe) . 18 

Station 2 (Sierra Zuluaga) 31 

Station 2 (Sierra Zuluaga) 40 

Station i (Jaguey) 30 



Cerritos de los Calzones 20 

Cedros 34 

Apizolaya 42 

Lomerio de los Zorrillos 49 

Encarnaci6n 26 

Average rate for all 30 



It will be seen that these figures, made at different times on material 
from different localities, check each other fairly well. As said before, the 
average rate of growth thus deduced is probably somewhat high. The 
rate undoubtedly increases toward the fifth year, and a somewhat more 
rapid rate is then maintained during a few years, say from the fourth to 
the seventh, during which the total height of the plant increases at a 
greater rate than before or after. Usually during the second or more fre- 
quently the third year a set of branches start their growth, and with this 
the weight increases more rapidly. What this weight may amount to in 
four years is shown by 3 thrifty plants taken on the Lomerio de Zorrillos. 
These made growth as follows: 

Table t.^. 



1905. 


1906. 


1907. 


1908. 


Dry 
weight. 


fnm. 


mm. 


mm. 


mm. 


grams. 


10 


35 


60 


10 


12 


20 


50 


84 


40 


30 


20 


40 


160 


20 


30 



Reproduction. 



81 



Hence we may conclude that the weight gained in four years' growth 
can scarcely exceed i ounce, and probably seldom amounts to that. 

The following are measurements (in millimeters) from rapidly grow- 
ing plants from Station 2, collected in January, 1908: 

Table 34. 



Plant. 


1902. 


1903. 


1904. 


1905. 


1906. 


1907. 


Notes. 


•{ 




mm. 
40 

40 


mm. 

30 

25 


mm. 

94 

76 


mm. 
32 

35 


mm. 

40 

20 


\ Average height from base of 1904 
j growth,2iomm."Weighti7gms. 


'{ 






16 
16 


84 
80 


65 
100 


45 
50 


\ Average height of twigs 194 
f mm. Weight 12 gms. 


3 






17 


34 


67 


10 


Habit strict, with short branches 
above. Weight 5 gms. 


4 


20 


30 


40 


70 


25 


25 


Not less than 6, possibly 7, years 
old. Height 200 mm. Dry weight 


1 












47 gms. 



It is of interest that plant 4, though a slower grower in height than 
I, made weight about twice as fast. This is due to the larger number 
of twigs. Plant 4 may be regarded as an expression of the best results 
which may be expected in this station. We may therefore conclude that 
the weight of 4-year-old plants will not on the whole exceed 1 5 grams or 
0.5 ounce, and that the maximum weight for a 6-year plant will not 
exceed, say, 45 grams or 1.5 ounces. 

RATE OF GROWTH IN MEDIUM-SIZED PLANTS. 

As in the case of seedlings, the annual accretions of growth have 
been measured only when suflficiently clear for certain recognition. The 
last 2 to 5 or more years' growth was measured, according to the visi- 
bility of the markings. Several hundred measurements were made in 
all, of which the summaries and averages alone are given in table 35. 



Table 35. — Average amount of growth per year in the localities indicated. 



Locality. 



Sierra Candelaria 

Station i (Jaguey) 

2 (Sra. Zuluaga) 

4 (Sra. Guadaloupe). 

5 (Sra. Guadaloupe). 
Cerritos de los Calzones 



Average 
amount of 
growth. I 



Locality. 



44 
30 
31 
41 

38 
'82 



Cedros 

Apizolaya 

Lomerio de los Zorrillos 

Encamacion 

Caopas 

Average of all . . . 



Average 

amount of 

growth. 



56 
42 

49 
28 

31 
43 



1 Note. — The plants in this locality showed very rapid growth in 1906, explainable by the rain and 
by their having been previously cropped back. The branches were few in number, so that the plants 
though relatively tall, were very light in weight. This figure would therefore better be thrown out of 
account, in which case the average falls to 38 mm. per year. The datum for Station 2 has been checked 
up by a later count, 26 measurements giving an average of 32 mm., and at this pomt it may be said 
that the data above given are collated from measurements made at different times, results being used 
as checks, the one on the other. 



82 Guayule. 

In addition to data obtained by observation of external marks, a 
number of measurements of field plants were made by the usual labora- 
tory method of marking the stem with India ink. The results of these 
observations are here given: 

Station 2, quadrat 3. 6 twigs marked at the tip with a drop of ink, Jan. 6, 

1908. Growth commenced Apr. 28. Last observation Apr. 3, 1909. Measure- 
ments as follows, in mm.: 60, 75, 70, 75, 65, 50. Average amount of growth for 
season, 66 mm. 

Station 1. 5 plants marked Jan. 3, 1908. Last observation made Apr. 3, 

1909. The mark had been destroyed on 2 plants. The total amounts of growth 
for the 3 remaining were 30, 18, and 35 mm., making an average for the 3 of 28 mm. 
All growth was subsequent to May 29. 

Station 3. Dec. 31, 1907. 3 marked plants showed an average growth of 
I to 2 cm. A seedling slightly pruned showed 2 cm. new growth by July 15. The 
rate of growth in all plants at this station was small in 1907. 

Station 6, a low gravelly ridge in the playa, Burrita. 4 plants marked Oct. 11, 
1907. On Jan. 11, 1908, 2 plants showed i mm. and 2 plants 2 mm. growth each. 
The total amount of growth till Aug. 21, 1908, was 13, 20, 20, and 10 mm., or an 
average amount of 18 mm. This is a locality of conspicuously slow growth. 

The average amounts of growth observed in marked plants for the 
season of 1907 were, therefore, 66, 28, 20, and 18 mm., making a grand 
average of growth of 31 mm. The average is lower than the one above 
deduced from measurements of field plants, but as three of the stations 
suffered severely from drought in 1907 the rate of growth was probably 
rather low. Our data on the whole indicate that the rate of growth of 
guayule in the field lies somewhere between 30 and 40 mm. annually. 
This general conclusion can scarcely be said to be too optimistic. It will 
no doubt be questioned by those who entertain different ideas of the 
rate of growth of this plant. The belief is current in many quarters in 
Mexico that growth is much more rapid, it being a common saying that 
after guayule has been cut the crop is reestablished in five years. Such 
surprising statements were made to me regarding one locality in particu- 
lar that I made special effort to obtain evidence. Although an attempt 
to visit the place, some leagues to the west of Escalon in Chihuahua, was 
frustrated, I succeeded, through the courtesy of some friends, in getting 
a number of plants, which, though of somewhat more rapid growth than 
usual, are not remarkable in a special degree. The plants were clean- 
limbed and smooth-barked, the effect of this more rapid growth. They 
bear evidence of a heavier rainfall as compared with plants from Zacate- 
cas, but this appearance is due in part to the fact that they are of two 
different types; in one the foliage shoot is abruptly terminated at the 
base of the peduncle; in the other the shoots taper out into the peduncle 
after the fashion in mariola. The branches in the latter are thin, die back 
readily and often for a good distance, and in these plants have some of 
the characteristics seen in the stems of irrigated plants. I give measure- 
ments of the few plants, which came to me for study, in detail (table 36). 

The average amount of growth of each plant for the years indicated 
is: plant i, 30 mm.; plant 2, 41 mm.; plant 3, 37 mm.; and for all the 
twigs on 3 well-developed plants of the first-mentioned type, viz, with 
abruptly ending foliage-shoots, it is 37 mm. The data are instructive in 
that they point to a " fat " year preceding two "lean " years, namely, 1907 
and 1908. The rate of growth, however, compares very closely with 
that derived from material from other localities. 



Reproduction. 
Table 36. 



83 



Plant No. 


Branch 
No. 


Amount of increase in stem length for — 


190S. 


1906. 


1907. 


1908. 


No. I, 85 cm. tall, dry weight 5 lbs. 
5 oz., symmetrical, well developed. 

Average 


4 

I 5 


mm. 
62 

45 


mm. 
41 
42 

57 

26 
41 


mm. 

8 

18 

10 

25 
II 


mm. 

8 

10 

4 
18 
16 


53 


14 


II 


No. 2, 60 cm. tall, dry weight 2.5 lbs., 
irregularly developed. 

Average 


I 
2 
3 

6 

7 
8 






117 

80 
55 
65 
80 

57 


10 
86 

77 
22 
16 
10 

23 
8 


20 

15 
28 
18 
20 
10 

13 
10 




76 


31 


17 ' 


No. 3, 35 cm. tall, dry weight 9.5 oz., 
10 yrs. old, well developed, symmet- 1 
rical. 

Average 


I 
2 
3 
4 

I 5 


50 
50 
55 
60 

25 


60 
70 
62 
70 
90 

70 


17 
20 

17 

7 

15 


25 
10 

25 
8 

15 


48 


15 


17 









Table 37. 








Plant No. 


Branch 
No. 


Amount of 


increase in stem-length 
for— 


1906. 


1907. 


1908. 


No. 4, seedHng 23 cm. tall, weight 10 gms. 

No. 5,38 cm. tall, weight 48 gms 

Average 


tnm. 

I 


mm. 

57 


mm. 
90 


mm. 
24 


I 

2 

. 3 


60 


48 
36 

45 

43 


90 
46 
70 

69 


60 


No. 6 

Average 


I 

2 

3 

4 

■ 5 

6 

7 
8 

. 9 


50 
60 

65 
20 

30 
80 

35 

47 


15 
30 
23 
15 
12 
8 

15 
30 
25 


50 
60 
40 
60 
6 
30 
45 
30 
75 


19 


44 




Average for all years 


37 I 





84 



Guayule. 



Table 37 gives measurements for the years indicated of 3 "spindling" 
plants, which grew rapidly in height but did not develop branches and 
therefore weight. 

Plant 7 was 50 cm. tall and weighed 153 grams, ragged, but showing 
abnormal development on certain shoots. The last three years of its 
growth showed accretions, a side-shoot starting low down, of 110, 200, 
and 60 mm. The upper shoots appeared quite similar to those of the 
other plants, but were more or less damaged, so that one could not get 
satisfactory measurements. 

The conclusion one is forced to draw from a survey of the above 
tables is that in a certain proportion of the plants in the locality referred 
to the rate of growth per year approaches closely to 50 mm. In these 
plants, however, the branches are thin, and the plants are not well devel- 
oped nor heavy for their size, so that, economically considered, there is 
nothing gained. Whether the differences in rate of growth are connected 
with racial differences in the plants is discussed elsewhere. 

RATE OF GROWTH IN IRRIGATED PLANTS. 

A considerable number of plants were under observation for the 
whole of two growing-periods, during which time they were freely irri- 
gated' and grew rapidly, at a rate close to the rhythmic maximum. The 
average rate of growth for the two years was very close to 25 cm. per 
year, so that a spread of a meter was attained by nearly all of the plants. 
The character of the growth is described elsewhere, but the fact here 
stated indicates very clearly that plants in the field may never be ex- 
pected to reach this maximum. The greatest growth of stem-length in 
field plants for one year, 200 mm., was seen in a very few twigs and in 
shoots favorably placed, the rest of the plant failing to behave similarly. 

The weight attained in two seasons by irrigated plants growing 
from small butts after transplanting is upwards of 2 pounds, or slightly 
over. The fresh weight of a large plant was 4.5 pounds. Another col- 
lected at the same time weighed fresh 3.5 pounds, and shrank in drying 
to I pound 10 ounces. The dry weight of two others was 28 and 32 ounces. 

On the other hand, plants under limited irrigation were grown at 
Caopas. I have examined three sample individuals of these, a large, a 
medium-sized, and a small plant. All of these failed to start promptly, 
and had been pollarded. The amount of growth made by them is shown 
in table 38. 

Table 38. 



Size of plant. 


Distance 

pollarded 

above 

ground. 


Amount of growth. | 


1908. 


1909. 


Total. 


Large 

Medium 

Small 


cm. 

40 
30 
IS 


cm. 
1 1 

16 to 18 

I 1 


cm. cm. 
9 30 

10 to 13 1 35 to 30 

8 21 








1 



' In 1907, through the winter until the following April. They were not irri- 
gated later, but received rain in the summer. They had a sufficient amount of 
soil-moisture for continuous growth. 




CQ 



Reproduction. 85 

The smaller amount of j^rowth in 1909 was due to the absence of 
irrigation, as elsewhere explained. It will be noted that the medium- 
sized plant responded best, which in general comports with our observa- 
. tions of the rate of growth of field plants. 

General Conclusions. 

The maximum rate of growth of guayule under irrigation is in the 
neighborhood of 25 cm. per year stem-length. The amount of growth 
between the field average and the maximum average for irrigated plants 
may be closely regulated by irrigation, to which the plant readily responds. 

FIELD PLANTS. 

When it is borne in mind that the total height of a plant is, except 
in young seedlings of strict habit, always less than the sum of its longer 
annual growths, because of (a) partial dying back and (6) the branching 
habit ; and when also it is remembered that numerous plants suffer from 
untoward conditions, either by the depredations of parasites or from 
poor soil-conditions, it is not far from the truth to say that the average 
annual rate of increase in height is 3 cm. A plant 30 cm. in height would 
therefore be 10 years old. Plant 3, above described (p. 83), which has 
undoubtedly a higher rate than 30 mm. per year, is, as certainly as may 
be estimated, 10 years old. As has been said previously, however, the 
important desideratum is to determine the period of life during which 
the increase in weight is most rapid, aside, of course, from the very young 
seedling stages, when the ratio of increase may be rapid, but the total 
weight very little. For the purpose of arriving at this information, I 
have assumed the rate 3 cm. per annum as a constant factor. A large 
number of plants have been weighed and measured, and the data thus 
derived have been correlated so as to obtain curves of increase in weight 
according to size (fig. 13). For the data the reader is referred to tables 
4 to 13. 

The curves have not been constructed for plants over 40 cm. in height 
for two reasons : the number of plants beyond this size is very much smaller, 
and, again, their age is too great to admit them to a practical considera- 
tion of rotation periods. Observations from which, in part, the tables of 
data used in the construction of the curves have been derived, all go to 
show that the first pronounced gain in weight is entered upon after the 
plant has reached a height of 30 cm. The average weight of plants of this 
height is somewhat over 5 ounces, ranging chiefly between 2.5 and 8.5 
ounces. The average weight of plants 40 cm. tall is, on the other hand, 
15 ounces or more. That is, the average weight is trebled in making the 
10 cm. advance in height beyond 30 cm. This is shown in the positions 
of the curves, which, however, present more irregularities than one would 
wish, in spite of the fact that they are based on measurements of several 
hundred plants. The greatest fluctuations in the curves are caused by the 
introduction of exceptional individuals, for where larger numbers are used 
the curves are more uniform. The exceptional individuals may be either 
"spindling" or unusually well-developed in point of ramification. 



86 



Guayule. 



Age and Height. 
For the purpose of controlling the above conclusion I have, as clearly 
as possible, made estimates of the ages of plants of various sizes, making 
no assumption as to the rate of growth, but being guided solely by the 
marks in each individual. The results are compiled in table 39. 





Table 


39. — Size, 


weight, a 


id closely 


estimated ages of guayule plants. 






Weight. 






Series. 


Height. 






Age. 


Remarks. 


Fresh. 


Dry. 




em. 


oz. 


oz. 


yrs. 




I 


35 




IO-75 


II 


Very well developed, symmetrical. 




30 




5-0 


8 


Medium, rather undersized. 




28 




4-5 


7(8) ! Normal. 




25 




4.0 


7 


Do. 




17 




0.875 


6 


Do. 


II 


51 


18 


II. 6 


14 


Y-shaped, symmetrical. "Macho." 




35 


8 


4.0 


10 i Do. 


III 


50 


12 


6.7 


10 to 12 i Y-shaped, symmetrical. "Hembra." 




30 


6 


3.25 


7 to 8 Do. 


IV 


66 


32 


24.0 


19 to 20 I Y-shaped, narrow-leafed type. 




40 


7 


4-5 


10 to 12 


Do. 


V 


40 


10 


6.0 


i5(?) 


Slow-growing, broad-leafed type. 




35 


5 


3.06 


12 


Do. 


VI 


50 


54 


36 


17 to 20 


Densely branched, spreading, in full 




45 


44 


29 


L\..a,L. 

16 to 18 1 (Note. Plants of series VI are the 




33 


16 


14 


10 to II 1 heaviest for their height of any. 




21 


6 


5 


9 


except very occasional plants such 




20 


5 


3.75 


8 


as that in plate 8, fig. B.) 




15 


1-75 


0.93 


6 




VII 


65 


64 


43 


20 


V-shaped, half-spreading type, dense- 
ly branched, symmetrical (plate 8, 
fig. B). 




55 


32 


18 


15 to 16 






50 


18 


IS 


13 to 15 






U 


8 


5.5 


9 






28 


6 


4 


8 






24 


3 


1.5 


7 






22 


125 





5 


6 




VIII 


55 


18.5 






15 to 16 


Y-shaped (plate 8, fig. A). 




30 


6 






II 


Normal shape for age. 




25 


I 






9 


Do. 




20 


0.875 






7- 


Do. 




14 


O-S 






5 


Do. 




4.5 


0. 1 






2 


Do. 



These data have been charted in the accompanying curves, corre- 
lating age and height (fig. 15, upper diagram). It is an obvious objection 
to the value of these curves that they are based, necessarily, on compara- 
tively few plants, but their value is enhanced by the individual treat- 
ment, since the estimate of age was made with great care. A fairly close 
correlation emerges, however, from the diagram, from which we see that 
plants of ID years of age have a height of about 30 cm., and those of 15 
years about 40 cm. 

An increase in height of 10 cm. over 30 or 35 cm. is correlated (judg- 
ing from the data of table 39) with a doubling at least of the weight 



Reproduction. 



87 



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HEIGHT IN CENTIMETEBS 

Fig. 13. — Curves correlating height and weight in the plants recorded in tables s to 7 and 9 to 13, inclu- 
sive. The approximate averages are indicated in the curve of averages. 



2« 
?2 
20 
18 

16 

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10 IS ?0 25 30 35 40 45 50 55 60 «J 

HEIGHT IN CENTIMETERS 

Fig. 14. — Curves correlating height and weight of the plants in table 39. 



88 



Guayule. 









































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Z)45«7a«IO 

ACE IN VCAR9 



13 14 19 16 17 IS l» to 





































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AGE IN YEARS 



IS 19 20 



Fig. is. — Upper diagram: Curves correlating age and height of plants in table 39. Lower diagram: 
Age and weight correlated. The same plants. 



Reproduction. 89 

(fig. 14). Fig. 13, on the other hand, indicates a greater increase, to 
nearly three times the weight at 30 cm. The average weight of a plant 
30 cm. in height is, according to fig. 14, about 7 ounces, but as the plants 
considered in this curve are normally developed or indeed considerably 
above the average, the average weight of a 30 cm. plant is probably 
nearer to that indicated in fig. 13, viz, 5 ounces. The mere ratio of 
change in weight is not peculiar to these dimensions alone. What appears 
from the data is that the weight of plants up to the height of 30 cm. is 
not great enough for economical harvesting. The increase in size and 
weight, however, is as great in the subsequent five years as in the previous 
ten, so that taking the crop at the end of ten years would give results 
only half as great as the returns of a fifteen-year rotation period. 

This conclusion is shown graphically by fig. 15, lower diagram, which 
indicates that the weight of a plant advances from about 6 ounces at 
ten years of age to 1 5 ounces at fifteen years. A considerable minus error 
in the estimation of ages might be allowed, and yet the increase indicated 
in the preceding paragraph would still be shown. It is fair to state, how- 
ever, that there is little chance for such error, as I have taken the pre- 
caution of being conservative when there was doubt. 

Fig. 15, upper diagram, indicates that the estimate of rate of growth 
used throughout, viz, 3 cm. per year, is very nearly correct. "We may 
therefore conclude: 

(i) That the average rate of growth of guayule per annum is about 
3 cm. 

(2) That the amount of increase in weight between the tenth and 
fifteenth years of its age is at least as great as that occurring during the 
first ten years; and that this further justifies, from an economic point 
of view, a fifteen rather than a ten year rotation period, aside from con- 
siderations which might be drawn from loss by death (p. 71), could we 
ascertain this accurately enough. 



CHAPTER V. 
ANATOMY AND HISTOLOGY. 

While the anatomy of the Compositas has been studied in much 
detail, beginning with von Sachs, followed by van Tieghem, Vesque, 
Vuillemin, Col, and less voluminously by other writers, that of the genus 
Partheniuni had, up to 1901, received no examination. In that year the 
plant which supplies the object of the present treatise came to the atten- 
tion of the French botanists MM. Fron et Francois (1901), who gave a 
brief account of the more obvious features of the anatomy of the stem 
and of the structure of the fruit. A more extensive paper was published 
in 1908 by Dr. H. Ross, who visited Mexico in 1907 and examined the 
guayule in the field, chiefly about Saltillo. In this paper an anatomical 
study of guayule was supplemented by brief reference to two other species, 
P. incanum and P. tomentosum. To both these contributions, as also to 
those of more general import, reference will presently be made. 

ROOT. 
PRIMARY STRUCTURE. 

The primary root is diarch. The two bundles of protohadrome, of 
spiral vessels, become early united by a centripetal development of vessels 
forming a primary plate, on either side of which stand the two protolep- 
tome strands. At this time the stele has a continuous pericambium and 
is surrounded by a well-marked endodermis, which may be recognized by 
the bands of Caspary and by the starch-content of the cells (plate 22, figs. 
6-8). The starch-grains are relatively large and are compound. Their 
persistence is variable, traces being visible for some months in some 
instances, e.g., in a root 4 mm. in diameter; in other cases they may have 
disappeared in a few weeks. Thus in a root 0.46 mm. in diameter, in 
which radial thickening of the endodermis had just commenced, starch in 
these cells fluctuates, there being now more and now less, apparently 
according to the draft upon it by the tissues. Without the endodermis 
lie three layers of cortical cells with extensive intercellular spaces, which, 
however, do not occur between the outer layer of cortical cells (the hypo- 
dermis) and the epidermis. 

SECONDARY STRUCTURE. 

The epidermis begins very early to break down, so that in a root less 
than 0.5 mm. in diameter the earliest peridermal divisions have set in. 
These do not usually occur in the outermost cortical cells, which here 
take on, in a weak fashion, the characters of an exodermis, as described 
for Cephalanthus and Tecoma by Holm (1907), but in the second hypo- 
dermal layer (plate 22, fig. 7). At this time growth commences in the 
■cortex, both radial and periclinal divisions occurring (plate 22, fig. 8). 
•Growth of the endodermis is concurrent (plate 22, fig. 6). Both radial and 
90 



Anatomy and Histology. 91 

tangential increase in size results in (i) extension of the radial dimensions 
of the so-placed walls. This extension is confined to that part of the wall 
between its outer limit and the band of Caspary, leaving this band in the 
same position as before (plate 22, figs. 6, 8). With this fact in mind the 
endodermis may be identified for a long time, indeed frequently till it 
is well-nigh expelled by secondary thickening. (2) Tangential growth is 
accompanied by cell-divisions in the radial direction, Casparian bands 
being formed in the new walls (plate 22, fig. 6). In particular positions, 
namely, opposite the leptome bundles, the earliest* resin-canals appear.^ 
These do not belong to the primary structure of the root, but arise second- 
arily in the endodermis (plate 22, figs, i to 5). Their mode of development 
is as follows: 3 or 4 adjacent cells divide by periclinal walls, thus bringing 
it abotit that two or three places occur where 4 cells lie with their angles 
adjacent. Here the walls split apart, making a simple, prismatic, inter- 
cellular space without demonstrably different contents; the adjacent cells 
divide radially, so that each canal has now 4 cells contingent upon and 
peculiar to it. Later, further divisions, roughly parallel to the early canal- 
walls, result in the canal consisting structurally of two layers of cells 
constituting a prismatic tube. It is seen that the endodermal canals, for 
so they will be called, are arranged in two groups, of usually two or often 
three or occasionally even four canals each, each group being placed oppo- 
site a primary leptome bundle. This relation was first described by von 
Sachs for Helianthus. The structure of these canals does not change, 
though there ensues some displacement of the cells in roots which have 
thickened abnormally without losing the outer primary tissues. The figures 
displaying the cell lineage will make this clear (plate 22, figs, i to 5). 

The growth of the endodermis may be followed till it is thrown off 
by the formation of cork within it. During its history it enlarges from 
a cylinder, of o.i mm. inside diameter, of 18 to 20 cells, to one of 3 mm. 
diameter, composed of hundreds of cells, or even to larger dimensions, 
before being finally cut out. Throughout the greater portion only radial 
divisions occur, though the cells increase in radial depth. In the region 
of the canals, however, the endodermal cells divide in a general periclinal 
direction, giving rise to two or even more irregular series of cells. 

In the ultimate condition of the endodermis and of the secondary 
cortex the walls of the cells are reticulately thickened (plate 22, fig. 15), 
so that in a root 2 mm. in diameter, of a field seedling, the endodermis 
may be followed all the way around with great ease, provided that the 
rubber has been previously extracted. The reticulations are the result 
of the oval form of the broad, shallow pores, which are somewhat crowded. 
They are more strongly developed in plants grown under normal condi- 
tions, as appears from the fact that in an irrigated seedling, with a root 

' Ross (1908, p. 25), in stating that there are only a few canals in the primary 
cortex of the root, does not make it clear that he refers to canals of endodermal 
origin. 

* Exceptions occasionally occur in which the canals in one half-circle of the 
hypocotyl do not approach on entering the root, and conversely, cases occur in 
which the grouping of the canals occurs in the hypocotyl, on one side of it. In 
other words, the root-structure is taken on at a higher level on one side than on 
the other. 



92 Guayule. 

4 mm. in diameter, the reticulations were much less marked or absent. 
The same condition is found in the definitive stem. 

This large size of the cylinder of tissue inclosed within the endoder- 
mis is attained only under an abundant water-supply and other condi- 
tions insuring rapid growth. Under such circumstances the structure of 
the canal itself passes beyond the normal definitive stage, and the cells, 
usually and normally eight in number in transverse section, may suffer 
further divisions as shown in plate 22, fig. 5, to an extent sufficient, to- 
gether with some displacement, to render it somewhat difficult to exactly 
delimit the structure. In a root of this size, viz, 4 mm. in diameter, the 
endodermis may still be readily recognizable (though irregular in charac- 
ter, being in part of two rows of cells) by the starch-content or by the 
Casparian spots, or both. The position of the endodermis is always clearly 
shown, other signs failing, by the primary canals. 

The physiological changes in the endodermis are of particular inter- 
est. Reference has been made to the variableness of the starch-content 
of its component cells. When grown under irrigation the starch may be 
seen in much larger plants than in those which have grown under normal 
or field conditions. In these growth is less rapid and the extension of the 
tissues correspondingly less marked. In these also the secretion of rubber 
ensues earlier and is correlated with the occurrence of drought. In such 
plants the cells of the endodermis, together with others to be noted below, 
secrete rubber, so that, in a small seedling with a tap-root 2 mm. in diam- 
eter, the cells of the endodermis will be found engorged with this sub- 
stance. The cells of the canals are especially noteworthy in this respect. 
By taking advantage of the effect of water upon the rate of secretion, it 
may be shown that the secretion of rubber in the endodermis takes place 
first in the cells of the resin-canals (plate 41, fig. 6). Thus, in the root, 
4 mm. in diameter, of a seedling which grew with great rapidity, the canal 
calls were half-filled with small droplets of secretion which reacted to al- 
kanet. The specimen had previously been freed from alcohol-soluble sub- 
stances, and there can, I think, be no doubt of the nature of the material 
in question.^ 

The behavior of the pericambium in the region included between the 
primary leptome and the endodermis differs from its behavior elsewhere. 
One finds, in a root 1.2 mm. in diameter, that the pericambial cells have 
enlarged radially and have in some cases undergone periclinal divisions 
and the daughter-cells further radial divisions (plate 23, fig. 4). The peri- 
clinal divisions suggest initial cork-divisions, but this is not the case, as 
both the radial divisions and the further behavior of the cells show. With 
a slight increase in thickening in the root, sufficient to attain 1.5 mm. in 
diameter, the cell-walls are a little thickened and a rearrangement has 
taken place. The cells have apparently been compressed between the pri- 
mary leptome and the endodermis, and, under suitable conditions, as in 
the specimen from which plate 22, fig. 5, was made, have secreted rubber, 

* I have noticed that reaction to alkanet, which is the same in all the cells 
at first, becomes in the canal-cells darker with time, the preparation having been 
kept in darkness. I have attributed this, with some doubt, to the greater proto- 
plasmic content of these cells. Great care was taken to extract very thoroughly 
with absolute alcohol, and, in a part of my preparations, with caustic potash also. 



Anatomy and Histology. 93 

though less than in endodermal cells with which they are in immediate 
contact. Further development sees the collapse of the pericambium cells 
(plate 22, fig. 6), and, as seen elsewhere, the primary stereome occurs in 
the primary leptome just within the pericambium. 

The primary cortical cells outside of the endodermis are also capable 
of secreting rubber. That they do so at all is contingent on the rate of 
growth of the seedling. If this is rapid enough to remove the cortex 
before drought sets in, no appreciable secretion will have occurred. If, 
however, the rate of growth is lower, so that for the greater part of a year 
the tissue in question remains functional, the inner cells at least may be 
found densely filled with rubber. In the root, 5 mm. in diameter, of a 
field seedling fully a year old, the following measurements (along a radius) 
were made, from which an idea of the amount of primary cortex remaining 
active may be had: Wood, 1.4 mm.; secondary cortex, 0.64 mm.; pri- 
mary cortex, 0.15 mm.; cork, 0.27 mm. 

Early Secondary Changes in the Stele: (Hadrome). 

With the completion of the primary hadrome plate there ensues a 
centrifugal development of this tissue by the direct transformation of the 
protogenic cells adjacent to the middle part of the plate. The increase of 
hadrome extends along all radii except those lying near the plane of the 
primary plate, but usually rather less rapidly toward the primary lep- 
tome, so that in transverse section there appear two wings, so to speak, 
of hadrome. This is protogenic, but is added to quite soon by the activity 
of a cambium which first becomes apparent within and close to the pri- 
mary leptome bundles (plate 22, fig. 10), and extends toward and finally 
around the outer edges of the primary hadrome (plate 22, figs. 9 to 10). 

Up to this point in the development of the stele nothing exceptional 
is seen. The only question which has been raised is in regard to the pre- 
cise origin ^ of the earlier formed secondary hadrome elements, whether 
this is by means of the cambium which arises on the inner surface of the 
primary phloem, or is directly from protogenic elements lying adjacent to 
the primary hadrome plate. The evidence from the material here under 
discussion is that the latter is the case. 

Now, however, a behavior ensues which is somewhat unusual. Two 
independent mestome strands of (at first) a single radial series of vessels 
and a very small leptome strand arise, each, usually, in immediate con- 
tact with the primary trachea (plate 22, figs. 9 to 11). The emergence of 
a secondary root disturbs the exact position so that the earliest vessels 
may be somewhat removed from the primary hadrome. A similar condi- 
tion has been observed by me in Lamimn ample xicaule, and by Petersen ^ 
in other Labiatae. 

It is necessary to note that although these mestome bundles are ver- 
tically below the two lateral cotyledonary traces, we shall presently see 
that they are independent of these and have no connection whatever with 
them. 



* De Bary, Comparative Anatomy of the Phanerogams. 

^ I have not seen this paper, but am informed by Dr. Holm. 



94 Guayule. 

The appearance of the stele is now suggestive of a tetrarch structure 
and is as follows: In a plane at right angles to the primary hadrome 
plate lie the two leptome bundles, between which and the hadrome plate 
the primary cambium lies. In a plane coincident with the hadrome plate 
and just beyond its edges lie two small secondary mestome strands, 
formed independently. These, which I shall here call the intercalated 
strands, lie, therefore, in a plane between the two broad primary medul- 
lary rays (plate 22, fig. 10). The isolated condition of the intercalated 
bundles is, however, very transient, since the parenchyma rays between 
the axial hadrome strand and the small intercalated bundles are soon 
bridged over, the whole, save as mentioned in the following paragraph, 
coalescing to form a single axial strand of hadrome. Additional second- 
ary bundles are intercalated between those already present, at first from 
the four angles of the hadrome wings, so that in a tap-root 1.2 mm. in 
diameter before me (plate 40, fig. i) there appear 8 bundles, though it 
must be said that the appearance of the stele in roots of the same size is 
not by any means uniform. 

The closure of the hadrome wings by meeting the xylem of the inter- 
calated strands is not complete, and thus are left two islands (analogous 
to medullary spots) of unlignified cells about the edges of the primary ha- 
drome plate (plate 22, fig. 11). The outlines of these islands are quite 
irregular, and they may ultimately become compressed or lignified, so 
that it is only with diflficulty that they may be recognized. In thin roots, 
as especially in the fibrous laterals, the wood cylinder is very compact, 
and may have no parenchyma rays. In such also the secondary changes 
in the cortex are less extensive, and the pericycle is much compressed. 

Later Secondary Changes: (Cortex). 
With age the walls of the cortical cells become somewhat thickened 
and pitted. The intercellular spaces are very regular in shape, and regu- 
larly disposed. In a tangential section they appear very uniformly len- 
ticular (plate 28, fig. 5). 

Stereome and Secondary Canals. 

Aside from the secondary increase of wood and bast, the appearance 
of stereome and of secondary canals has to be mentioned. That stereome 
which appears in connection with primary tissues only may properly be 
spoken of as primary. Of this there are but two slender bundles (plate 
22), fig. 6), which consist each of a few (less than a dozen) slender, very 
thick-walled elements buried in a mass of material derived from the pri- 
mary leptome by the swelling of the cell-walls till the lumina become 
indistinguishable. 

The method of origin of these sclerenchyma cells is difficult to deter- 
mine, and will be discussed beyond. 

The pericambium appears to be continuous, but so far as I am aware 
the formation of stereome does not involve the cells of this layer. The 
configuration of the cells of the adjacent secondary cortex is at this time 
(a root 2 mm. in diameter) very curious, the walls having been distorted, 
as if by stretching in a radial direction from the primary sclerenchyma as 
a center. This result would seem, however, to be due to compression by 



Anatomy and Histology. 95 

the growth of a mass of secondary stereome which arises within the pri- 
mary stereome and is removed from it, in a root of 2 mm. diameter (plate 
23, fig. 6), by about 35 microns toward the center. 

This secondary stereome strand is larger than the primary strand 
and becomes an obvious structural feature. For this reason, and on ac- 
count of its close proximity and its relative position to the primary canals, 
it may very easily be mistaken for the primary strand. Its cells, however, 
are larger, and it arises in connection with secondary leptome, and not in 
relation to the proto-leptome. For this reason its position is more variable 
than that of the primary strand and may suffer tangential displacement 
(due to unequal development of the root), as shown in plate 23, fig.i ; and 
further, for less obvious reasons, the stereome may not occur at all. 

Other secondary stereome strands develop, if at all, always in con- 
nection with the leptome, as stated for the stem by Fron and Frangois 
(i9oi)andby Ross (1908). The particular mode of origin will be discussed 
later. Each series is circular (plate 23, figs, i and 2), as all the members of 
a series arise normally at the same time, though the series may be more or 
less discontinuous, owing to unequal development as between the mem- 
bers of the series. In seedUngs grown rapidly under irrigation the amount 
of stereome development is usually notably less than in field seedhngs or 
in others grown slowly. 

Secondary resin-canals (plate 22, fig. 13) arise within the secondary 
leptome in close proximity to the cambium, and in the manner described 
by Ross for the canals of the stem. They consist at first of two tangential 
rows of cells scarcely distinguishable from the cambium from which they 
arose, though quite early they may be recognized by their larger size and 
the dense protoplasmic contents which at first show by their reactions 
merely their protoplasmic nature. Ross (1908, p. 260), however, says 
of these canal-cells: "Die den Kanal auskleidenden Zellen sind durch 
dichtes Protoplasma ausgezeichnet , das sicli mit Chlorzinkiod dunkel- 
braungelb, mit Alkannin intensiv rot farbt, wahrend sonst das Leptom 
hauptsachlich starkereichen Zellinhalt fuhrt." 

My own observations difter from those of Ross in that the cell con- 
tents, when very young, do not react to alkanet as described by him. 
Very soon after the two cell-rows begin to split away, minute globules of 
a secretion begin to appear, and these indeed take on the intensive red 
color of the reagent. This is considerably in advance of the same appear- 
ances in the adjacent cortical cells. Preparations treated with alcohol to 
dissolve out the resin or oil, which might be said to occur, show this very 
clearly, and further treatment of the same preparations with benzole 
shows that these intensively staining masses are dissolved out by that 
agent, and, in the absence of evidence to the contrary, must be regarded 
as rubber. In the secondary canal-cells, therefore, as in the wall-cells of 
the primary canals of the root, occurs the earliest appearance of rubber, 
the secretive activity extending progressively from them to the surround- 
ing tissues, and more rapidly in the primary cortex. It is worthy of 
especial note that rubber occurs in the wall-cells of canals which normally 
contain, in the meatus, the resin characteristic of the guayule plant. This 
point calls for discussion, which will follow later. 



96 Guayule. 

The leptome of the root does not show any starch-content in the 
sieve part, though it occurs apparently erratically in its parenchyma and 
in the cortical cells adjacent also to the leptome and to the resin-canals. 
It is also to be found in the endodermal cells close to the primary canals, 
and occasionally elsewhere, in a 4 mm. diameter root of a field plant. 

If, however, we examine a plant grown with an abundance of water, 
in which the secretion of rubber has taken place only in minute quantities 
and this in the wall-cells of the resin-canals, an important physiological 
relation between starch and the secretory canal-cells is indicated. In a 
root 4 mm. in diameter, which developed in about three months, the dis- 
tribution of starch and its quantity are very striking. It is present abun- 
dantly (a) in a broad, irregular radial band of cortical cells extending 
from the primary resin-canals, {h) in a narrow and somewhat irregular 
circular band midway the secondary cortex, and {c) in marked quantities 
in the cortical cells adjacent to the definitive resin-canals. It is not pres- 
ent in the leptome adjacent to the young resin-canals. It would therefore 
seem probable that the presence of starch in marked quantities near the 
resin-canals is related either to the secretion of rubber by the wall-cells 
especially, or to the secretion of resin. The familiar case of Pinus, in 
which starch occurs near the resin-canals, suggests the latter. 

The earliest appearance of rubber, which is secreted by the paren- 
chyma of the cortical rays and of the cortex, aside from the cells of the 
resin-canals as above noted, is to be seen in the innermost cells of the rays, 
and synchronously in the outermost cells of the primary cortex, or, if that 
is absent before secretion begins, of the secondary cortex. This fact is 
beautifully shown in the tap-root of a seedling from the field, probably 
less than one year old, collected on July 14, 1908, and measuring 2 mm. 
in diameter. In this specimen the cells of the primary cortex were com- 
pletely filled, as also the outer cells of the secondary cortex, there being 
progressively less and less secretion toward the center of the root. The 
opposite relation was shown by the parenchyma rays, in the cells of which 
the amounts of rubber were found to be progressively less and less, as one 
proceeded from the center outward (plate 23, figs. 3,7; plate 40, figs. 2 to 
4). In a still younger seedling, perhaps three months old, about 1.2 mm. 
in diameter, rubber is to be seen only in the cortical cells adjacent to the 
primary canals and in the few innermost cells of the cortical rays. The 
amount is so small here that, while it may readily be seen with the eye, 
the photograph does not differentiate it. 

HYPOCOTYL. 

PRIMARY STRUCTURE. 

The primary cortex consists of six layers of cells, including the 
endodermis. The epidermis becomes rather strongly cuticularized and 
many of the cells are papillate, or, more correctly speaking, form short, 
round-ended trichomes, which are usually one-celled, though two-celled 
trichomes are found in a few instances (plate 23, fig. 9). The angles of- the 
cortical cells adjacent to the inner faces of the epidermal cells are coUen- 
chymatized, but in deeper layers this character is not present. Chloro- 



Anatomy and Histology. 97 

plasts, few in number, however, are present in the cortical cells. The 
endodermis is well marked and contains a good many large starch grains. 
The Casparian spots are readih' recognized. 

The stele (0.18 mm. in diameter in a hypocotyl 0.53 mm. in diameter) 
is, at an early age, tetrarch above the zone of transition to the root. The 
four bundles are received into the hypocotyl in pairs, one pair from each 
cotyledon, in which they constitute the median trace. After reaching the 
lower part of the lamina they unite, as they do in the lower part of the 
hypocotyl (in the "collet"), to form a single bundle. 

In addition to the median paired cotyledonary traces, there are 
delivered into the hypocotyl four lateral traces which meet in pairs to 
constitute two bundles which pass inward and downward. Each takes a 
position, the one on one side of the stele, the other on the other, in a ver- 
tical plane at right angles to that which divides both cotyledons. They 
are the "faisceaux lat^raux " of Dangeard (1889, p. 85). So far, then, this 
plant satisfies the "cas secondaire" of his root type with two bundles, 
found in the Compositae and certain Ranunculaceae. According to Dan- 
geard, however, the behavior of these bundles is, to use his own words, 
as follows: "Les premiers (f. median) se comportent comme dans le cas 
general;^ les lateraux s'anastomosent plus ou moins longuement avant de 
rejoindre le median vers le bas." 

If I interpret Dangeard's statement correctly, we should find that 
the lateral traces (plate 24, figs. 2 to 5, 12) anastomose with the median. 
This, however, I do not believe to be the case. By following the figures it 
will be seen that the lateral traces are to be seen in the upper part of the 
hypocotyl, but end rather soon. The broad medullary ray between the 
pairs of median bundles is then unoccupied, and remains so till the cauline 
bundles encroach upon it. Between these cauline bundles, at the proper 
level, the slender end of the lateral cotyledonary trace may be seen, quite 
single and separate from them (plate 25, fig. 10). In the diagram (plate 
24, fig. 13) the fused lateral traces are represented as being much shorter 
than in that given by Dangeard for Catananche lutea. 

In types with a tetrarch root-structure this trace passes downward 
and articulates directly with two of the primary hadrome strands. This, 
e.g., occurs in Caulophyllum thalictroides (Butters, 1909) and in numerous 
other plants cited by Dangeard {I.e.). 

The intervals between the lateral and median bundles are occupied 
by two cauline traces, or, more properly speaking, by one (lateral pro- 
phyllonary) and a half (of the corresponding median prophyllonary) 
traces. There thus appear in the higher levels of the hypocotyl: 

(i) 8 cotyledonary traces, viz, 2 pairs of half -median traces; 2 pairs 
of lateral traces. 

(2) 8 prophyllonary traces, viz, 4 half-traces, a half-trace on each 
side of the cotyledonary laterals; 4 lateral traces, one on each side of the 
cotyledonary median pairs. 

Passing down, the i^ cauline bundles in each cotyledonary median- 
lateral interval fuse with each other and then with the adjacent median 
trace. Below the level of this fusion the tetrarch structure is assumed, 

* That is, as above described. 



98 Guayule. 

the paired cotyledonary median bundles becoming somewhat separated. 
The separation is, I beHeve, due to the rapid enlargement of the adjacent 
parenchyma-cells, so that the secondary elements become, in the lower 
part of the hypocotyl, definitely dissociated, leaving the primary ele- 
ments occupying the position of the primary hadrome elements of the 
root. The primary hadrome plate of the root lies, then, in the plane of the 
cotyledons (Dangeard, I.e., p. 87). In making the approach to the root 
the leptome masses revolve, two in one direction and two in the other, 
until they meet, two and two, to form the diametrically opposed leptome 
masses of the root' (plate 24, fig. 2); above, these same leptome masses 
pass entirely into the cotyledons, with the corresponding hadrome masses, 
and not into the stem. The continuity of vascular tissues between the 
stem and root is established secondarily. 

The above account of the structure is incomplete in that the presence 
of an originally single tracheal vessel, extending from within the cotyle- 
don downward through the hypocotyl into the root, has not been pointed 
out. This trachea {trachee primitive of Vuillemin, 1884, p. 183) consti- 
tutes a center of development, identical in the hypocotyl and tap-root, 
for the primary hadrome. It is unnecessary to recount the arrangement 
of hadrome in these organs, but it is pertinent to insist on the initial 
centripetal formation of new hadrome elements. The dissociation of the 
hadrome elements in the hypocotyl — strictly speaking, only in the upper 
portion — is due, as Vuillemin has stated (1884c, p. 181), to the rapid 
development of parenchyma, and is analogous to the secondary splitting 
apart of the wood cylinder in the same organ by the growth of the con- 
junctiva. In consequence of this interpretation Vuillemin speaks of the 
paired bundles as "les deux moities du faisceau," which are secondarily 
separated by "a medullary ray." The peculiar orientation of the paired 
bundles represented (but frequently not referred to) by many observers 
(van Tieghem, Gerard, Dangeard, Goldsmith, Ramaley) is thus, properly 
I believe, explained.^ 

Primary Resin-Canals. 

These arise in the endodermis, as in the root, as a single canal directly 
opposite each primary leptome strand (plate 25, figs, i to 6). The struc- 
ture of the canal is similar to that in the root, and consists in its definitive 
form of eight cells in transverse section. The course of development is 
not so regular as in the primary-root canals, the meatus being ultimately 
more cylindrical. Not infrequently the earlier divisions do not all take 
place, so that three instead of four cells line the meatus (plate 25, fig. 2). 
As these canals pass into the root they pair off, each pair coming to occupy 
the position already described, namely, opposite each of the two primary 
leptome bundles. When more than two canals are encountered in this 
position it is the result of branching. Occasionally both branch either 
once, or frequently twice, giving rise at length to four or even six canals, 
though more frequently three or four only occur. 

' The above account may be applied to Parthenium incanum, Lactuca, and 
Helianthus in its main outlines, and is a type, I believe, of wider applicability than 
usually supposed. 

* As for the rest of Vuillemin's views, regarding the nature of the hypocotyl 
and cotyledons, I will say only that they appear to me somewhat strained, and 
far less in accord with the course of development than those of Dangeard (1889). 



Anatomy and Histology. 99 

Stereome. 

The primary stereome arises early in the hypocotyl, as four slender 
bundles just within the canals, within the outermost part of the primary 
leptome strands. Occasionally, also, endodermal cells adjacent to the 
canal may undergo sclerosis, both in the hypocotyl. stem, and leaf (plate 
25, figs. 5, 6, 11). 

That unmodified pericycle cells lying just within the endodermis 
and opposite the leptome become sclerosed seems possible (plate 25, fig. 6), 
but doubtful. I find that the pericycle is quite frequently interrupted 
(plate 22, fig. 12), in which event the stereome must arise in the primary 
leptome. Its further development is contributed to chiefly by elongated 
elements in the leptome, and a few elements are sometimes derived also 
from the leptome parenchyma. Nearly all the elements (except those of 
parenchymatous origin) which play this part enlarge greatly (plate 25, 
fig. 4) and cause marked displacement in the surrounding tissues. Vuille- 
min (1884a) has described stereome arising, in the Compositae, in the sec- 
ondary, but not in the primary leptome, in Achillea, Artemisia, Anthemis, 
and Leonto podium. From my own studies I am forced to the conclusion 
that this takes place in the primary leptome. 

SECONDARY STRUCTURE. 
The prophyllonary bundles, above referred to, arise in the intervals 
between the cotyledonary bundles, before the establishment of inter- 
fascicular cambium (plate 24, figs. 2 to 5, 12). This, when complete, 
incloses the cotyledonary hadrome, and there is thus established the basis 
for the imposition, on the primary stele, of secondary, true stem topog- 
raphy. It may be pointed out, however, that the cambium does not 
lay down secondary hadrome in all cases in immediate contact with the 
primary elements. Thus, in the radii of the median cotyledonary traces 
(the elements of which do not of course pass beyond the cotyledonary 
node) secondary hadrome arises which descends from the epicotyl (plate 
25, figs. 10 to 106). Between these there is frequently a hiatus which 
delimits them readily to the eye, if the secondary changes have not pro- 
ceeded too far. Nevertheless, though the morphological separateness of 
the primary and secondary hadrome — and also leptome — is clear, the 
peculiar topography, the curved outline of the secondary hadrome as 
seen in transverse section, indicates an as yet entirely unanswered ques- 
tion as to the immediate cause of this. As it is purposed to compare 
ecological types, further detail will be considered in the following para- 
graphs. 

Field Plants. 

The pith in a specimen about 1.8 mm. in diameter displays at an 
appropriate level two gaps (plate 25, fig. 7), each in the position of a 
primary medullary ray, containing the primary bundles' constituting the 
lateral leaf-traces, while its transverse outHne still reflects the vascular 
topography of the primary condition. Surrounding the pith is a closed 
compact column of hadrome which is broken up radially into broad wedges 

' These undergo little or no secondary thickening, except in a restricted region 
below the cotyledonary collar. 



100 Guayule. 

by secondary parenchyma rays (plate 25, fig. 7). The primary rays are 
for the most part entirely closed, though two of these are suggested by 
the topography of the pith, as above indicated. No more resin-canals 
have appeared. The primary stereome bundles have extended inward 
by the transformation of the primary leptome, and the primary resin- 
canals are still present. Small secondary stereome strands are present 
on the outside of several other bundles, as indicated in plate 25, fig. 10. 
The endodermis is recognizable by its starch-content. The primary cortex 
is much reduced, its tissue having been sacrificed to the development of 
a thick cork, the original peridermal divisions of which take place in the 
outermost cortical layer of cells (plate 23, fig. 8). 

The seedling in question (plate 25, fig. 7) was less than a year old, 
probably four to six months. The epicotyl was 8 mm. long, with a few 
small leaves, and was collected on July 24, 1908. 

An etiolated seedling (plate 25, fig. 9) of the same diameter, with an 
epicotyl 2 cm. long and about three months old, shows a similar topog- 
raphy, save quantitatively. There is a weaker and more irregular devel- 
opment both of hadrome and of leptome. There is no additional stereome 
beyond the four primary strands. The primary cortex is thicker and the 
cork thinner. This seedling was supplied with abundant water and the 
shade of a muslin cloth, with the effect of producing responses correlated 
with a relative reduction of transpiration and to loss of water from the sur- 
face of the stem. The greater leaf -area, together with a more slender axis, 
results, however, in a greater transpiration stream relative to the diameter 
of the wood cyHnder, with histological results to be noted beyond. 

An irrigated plant (plate 25, fig. 8) of slow growth, one which was 
plentifully supplied with water, exposed to full illumination, but limited 
in the spread of its roots, is very instructive in this connection. Under 
these conditions we must assume a strong transpiration stream, at least 
stronger materially than is usually the case in field plants. The specimen 
had a diameter of 2.5 mm. and was not more than three months old, and 
on this account alone was therefore a trifle larger and more advanced in 
development than the preceding. In its cork development it resembles 
the field plant, and has suffered the same reduction of the primary cortex. 
In fact, in both cases one of the primary canals is just cut out by the peri- 
derm. The deeper medullary rays communicate with the pith, indicating 
secondary enlargement of the latter. The amount of wood as compared 
with the field plant is much greater relative to age, but somewhat less 
relative to radial measurement, and there is a relatively larger growth of 
the secondary cortex. Most remarkable is the large and irregular devel- 
opment of stereome. This irregularity is constantly associated with a 
plentiful water-supply and is an expression of a general tangential dis- 
placement of cortical tissues, as revealed by the later positions taken by 
the primary resin-canals and the obliquity of the leptome masses, the 
position of which predetermines that of the secondary stereome. 

Aside from the total quantity of hadrome, these three ecological 
types present histological peculiarities which are related to the transpira- 
tion stream. The number and size of the vessels in the field plant (plate 
26, fig. 2) are scarcely inferior to those in the irrigated plants (plate 26, 



Anatomy and Histology. 101 

fig. 4), while the etiolated plant (plate 26, fig. 3) has vessels somewhat 
fewer, but of more uniform size and notabh^ larger. The mechanical ele- 
ments of the wood are thicker-walled and somewhat smaller in the field 
plant (plate 26, fig. 5), and are nearly isodiametric. They are of much 
the same character in the other two, except that they appear more com- 
pressed tangentially, especially in the irrigated plant (plate 26, figs. 6, 7). 
The stereome also presents differences which are still more striking, 
aside from the relative amounts already spoken of. In the field (plate 
26, fig. 8) and etiolated (plate 26, fig. 9) plants the cells are closely set 
together, but are smaller on the whole, and in the field plant have smaller 
lumina. In the irrigated plant (plate 26, fig. 10) the shape and size vary 
greatly, the lumina are very small, and the intercellular material is much 
more extensive. The whole appearance leads to the impression that there 
is a good deal of distortion during development, so that the fibers are 
pushed about and disarranged, the tissue becoming less compact. If my 
view of the origin of the stereome is correct, the explanation of this con- 
dition may lie in a less complete transformation of the sieve-tissue into 
stereome. The collapse of the unsclerified cells would cause displace- 
ment, and the irregularities due to change in position and unequal growth 
of the stereomatic cells would ensue. The more slowly growing tissues 
are the more regular and the more compact. The stronger development 
of mechanical elements in irrigated plants, both in the cortex and stele, 
appears to be correlated with the larger growth of shoot, while the larger 
vessels of the etiolated plant indicate the greater proportion of transpir- 
ing surface (the leaf-surface) to the diameter of the stem. 

LATER SECONDARY STRUCTURE. 

As the hypocotyl approaches a diameter of 3 mm. a total movement 
outward of the whole vascular system (including the entire wood cylin- 
der) takes place, a result of the enlargement of the pith and adjacent 
parenchyma-rays tissue (plate 26, fig. i ; plate 28, fig. 3). The inner edges 
of the hadrome plates or wedges become more or less bent, because their 
edges are held together unequally by the original solid mass of early sec- 
ondary hadrome, which splits usually in four places, corresponding appar- 
ently with the primary parenchyma rays. These, therefore, are at first 
closed and later opened secondarily, as shown in the figure (plate 25, fig. 
8), in which the rupture of the xylem cylinder is beginning. In a field 
plant this expansion of the pith is also accompanied by a considerable 
tangential growth of the medullary rays. This circumstance, together 
with the relatively slower rate of growth of wood, brings about the result 
that in field plants (plate 25, fig. 7) the amount of wood is relatively less 
than in irrigated plants (plate 25, fig. 8) , and the medullary rays are wider. 
The thickening of the parenchyma rays is shown most strikingly in an 
etiolated seedling, the consequent rupture of the wood ^ in which is shown 
in plate 26, fig. i. 

As to the cortex, the growth has continued in all of its parts in such 
a manner as to still keep the primary cortical canals included within the 

* The separation of the young hadrome in succulent roots in this manner is 
well known. 



102 Guayule. 

living part. Two series of secondary canals have arisen in the hypocotyl 
of the size under consideration, whether the growth has been rapid or 
slow under irrigation (plate 29, figs. 3,4), or slow in the field (plate 28, 
fig. 3) ; but the total number of canals is greater in the irrigated plants, 
as would be expected in view of the more numerous wood plates. The 
radial depth of the cork has not increased in any appreciable amount in 
the field plant, but is more uniform than in a rapidly growing plant, in 
which it is relatively much thinner (plate 29, fig. 3). 

It is of interest to extend our comparison to rapidly and slowly grow- 
ing irrigated plants. The chief point of difference is seen in the much 
greater tangential development of sieve-tissue, and, later, of stereome, 
relatively to the size of the plant in slowly growing specimens (plate 29, 
fig. 4). This statement may be extended also to the mechanical elements 
of the wood, in which the libriform cells are of smaller diameter, have 
smaller lumina and are more cylindrical, implying a greater amount of 
intercellular cementing substances. The vessels too are of smaller diame- 
ter, and, though this is compensated for by their greater numbers, the 
capacity of the vessels in the more rapidly grown plant is considerably 
greater (plate 27, figs. 6,7). The phloem presents analogous differences, 
having in the slowly growing plant a structure denser and much more 
extended tangentially than in the rapidly grown plant, and in this, as in 
the character of the wood, resembling more closely the field plant (plate 
28, fig. 3). A still further difference, of more fundamental character 
morphologically, is the development, in slowly growing irrigated plants, 
of stereids in the pith (plate 29, fig. 4). So far as I have been able to 
observe, the stereids occur under no other condition in the hypocotyl, 
though, as will be shown, it occurs normally in the pith in the definitive 
stem (plate 29, figs. 5,6). 

The observations on the structure of the wood in the seedlings studied, 
regarding especially the water-carrying elements, are of peculiar interest 
as they stand in relation to those of Cannon (1905), who studied compar- 
atively irrigated and non-irrigated desert woody plants of eight species. 
His general conclusions, undoubtedly supported by his observations, are 
that " there can be no mistaking the fact that branches of irrigated plants 
(even if semi-irrigated only) are poorer in conductive tissue than branches 
of the same diameter of non-irrigated plants," but he says at the same 
time that this "is an unexpected condition." Further, "irrigated plants 
organize each year a larger amount of wood — which contains a relatively 
large amount of non-conductive tissue — so that it happens that non-irri- 
gated and older stems have more vessels than irrigated and younger" of 
the same diameter. 

For the reason that I found, to my surprise also, that some of my 
observations coincide with Cannon's, I venture to cite certain concrete 
instances, and state these, together with those already presented, in brief 
fashion, by way of instituting a comparison of our results: 

I . In field plants (the seedlings above studied) the vessels are as large 
as in irrigated plants of slow growth, or larger, and are slightly more 
numerous. The stems are of nearly equal diameter (plate 26, figs. 2,4; 
plate 27, figs. 6, 8). 



Anatomy and Histology. 103 

2. On the contrary, in irrigated seedlings of very rapid growth the 
vessels are much larger, though not quite so numerous, as in the plants 
mentioned under (i) ; but the total amount of wood is considerably greater 
relative to the diameter of the stem (plate 27, fig. 7). 

3. The terminal twig of a field plant of very large size, in which the 
amount of growth in any twig was very small in one season, is contrasted 
with an irrigated twig of rapid growth. The wood cylinders are equal in 
diameter; the vessels are somewhat larger in the secondary xylem of the 
field plant. But in the protohadrome the vessels are larger in the irri- 
gated plant (plate 27, figs. 4, 5). Both twigs of the same and last season's 
growth. 

4. Two twigs of about the same diameter of wood cylinder, one a field 
twig two years old, the other irrigated, one year old. The total number 
of vessels is greater in the field plant, and there are more large and more 
smaller vessels. In the protohadrome, however, the reverse as regards 
size is true. But the number of vessels in either year's hadrome in the 
field plant is probably the same as, or is less than, that in the irrigated 
plant (plate 27, figs. 9, 10). 

5. On the contrary, in another irrigated stem 6.5 mm. in diameter, 
the number and size of the vessels are enormously superior to the number 
and size in a field plant (plate 27, figs. 2, 3). 

6. The protohadrome in a field seedling of usual growth compared 
with that of an irrigated plant, before secondary xylem has appeared in 
either case. In the irrigated plant, in which growth is rapid, the elements 
in question are much larger (plate 26, figs. 11, 13). 

7. The protohadrome in a peduncle, through which there is, relative 
to its size, it can hardly be doubted, a very large transpiration stream, is 
composed of very large elements (plate 26, fig. 12). 

8. In an etiolated seedling (plate 26, fig. 3), in which the size of the 
stem remains small in relation to the total transpiring area, the size of the 
conducting elements is greater, and their numbers scarcely less, than in a 
field or irrigated seedling of approximately the same size of stem. 

9. In the tap-root of very rapidly grown seedlings the vessels are 
much larger and the amount of mechanical tissue much less. 

These observations are in part antagonistic, in appearance at any 
rate, to those of Cannon, and in part agree with them. They must there- 
fore be harmonized among themselves as well as with Cannon's. In at- 
tempting to cover all the cases with one explanation, we must not forget 
that the problem indicated is a complex one, inasmuch as the ratios of 
mechanical tissues in the two types enter into it. It will, however, suffice 
to speak of the conducting elements alone at the present moment. 

In stems of guayule of a given diameter in field and irrigated plants, 
the amount of wood is greater in the latter. In wood cylinders * of equal 
diameter the same holds true. This is due to (a) the smaller amount of 
cortex in irrigated plants and (b) the narrower medullary rays. We may 
assume that the growth in thickness of the stem is correlated with the 
growth of the shoot above. In the same period, the total amount of con- 

' Wood, medullary rays, and pith taken as a whole. 



104 Guayule. 

ducting tissue formed in irrigated wood is undoubtedly much greater than 
in that of field plants, but the amount of mechanical tissue is also greater. 
Putting these facts together, it seems reasonable to conclude that the 
capacity of the conducting elements is correlated with the maximum transpi- 
ration stream. The relative numbers, and therefore their size, depend 
primarily upon other conditions productive of the development of me- 
chanical elements. On comparing the shoots of field and irrigated plants, 
it is clear that the mechanical conditions in the latter are those under 
which mechanical tissue would be developed. The mere weight of the 
foliage alone would be expected to insure such responses. 

Advanced Secondary Condition of the Hypocotyl. 
In a more advanced stage of growth nothing of especial note, be- 
yond that already pointed out, presents itself for discussion. One point, 
however, is worth noting, namely, that the daughter and granddaughter 
cells of the cortex remain arranged in tetrads chiefly, thus giving the 
Avhole tissue the appearance of consisting of pairs and tetrads of cells. 
The original, but enlarged, intercellular spaces are very much in evidence 
(plate 28, fig. 4). Regularly shaped and disposed spaces, such as have been 
described for the root, do not occur in the stem. 

AGE AND STRUCTURE IN THE SEEDLING. 

Both popular and scientific discussion frequently turn on the corre- 
lation of age and structure in the guayule. Inasmuch as the hypocotvl is 
the oldest portion of the stem, it is worth while to indicate the structure 
of field plants of known age. A seedling from Station 2, which was less 
than one year old when collected in April 1909, with a stem 5 cm. long 
and 4.6 mm. in diameter at the base, shows in the hypocotyl the struc- 
ture represented in plate 30, fig. i. The living cortex (primary) is very 
sharply delimited from the cork on account of the rubber-content of the 
living cells. It will be seen that the specimen closely resembles the slowly 
grown irrigated plant above described, while in point of fact it is a plant 
of rapid growth for field conditions, being much above the average size for 
the locality in which it was collected. It is seen from this, what will in 
any event be understood, that all field plants are not alike, the water- 
supply varying at different times in different habitats, thus inducing at 
times growth quite similar to that which usually occurs under more 
favorable conditions. This seedling has, in addition to the four primary 
canals, three series of secondary canals. One below the average size, of 
the same age, with an epicotyl 8 mm. long, 2.4 mm. in diameter, has only 
the four primary canals. These are finally thrown out when a diameter 
of 6 to 7 mm. is attained, and therewith the whole of the primary cortex 
is lost. 

In a seedling of the same diameter three years old, it is possible to see 
three annual rings of wood, marked by the larger pores of the first growth 
of each season. There are in the same stem, aside from the four primary 
canals, three series of resin-canals, one in the primary and two in the 
secondary cortex, so that there are marks of three zones of cortex, the 
primary and two secondary, corresponding apparently with the three 



Anatomy and Histology. 105 

seasons of growth. Comparing the two cases, we find that the structure 
attained in the cortex by a seedling of one season may be the same as that 
attained in three years by one of slower growth, while the number of 
growth-periods is reflected , albeit frequently only indistinctly , by the wood . 
It is, however, generally true that the ring-structure may be made out. 

EPICOTYL. 

Seedlings partially etiolated by being grown under a muslin screen, 
in which the intemodes have lengthened, render the analysis of the tis- 
sues easier. The lowermost intemodes of such seedlings receive primarily 
six bundles (plate 24, fig. 5) from the hypocotyl, but the number is at 
once increased, so that immediately above the base eight or even more 
bundles may be counted (plate 24, fig. 6). The increase is more marked 
in plants w^ith short intemodes, and the primary condition is quickly 
masked. The development of the stereome which arises in the primary 
leptome is in the primary numerical relation, there being at first six 
bundles, opposite the median and lateral leaf-traces of the first two foliage 
leaves. These relations are shown very beautifully in a section taken 
from a seedling which had developed one-sidedly, and this is figured in 
plate 30, fig. 2. The relations of the primary cortical canals received from 
the hypocotyl are well shown also in this section. One pair of these 
accompanies the median leaf-trace of the first leaf, the other pair that of 
the second leaf. The third petiole may receive two or one, and this is true 
of all the earlier leaves as far as the tenth node, approximately. The 
primary condition, that in which two lateral canals occur, may recur even 
in later stages of development, but only infrequently. As they pass into 
the leaf one becomes smaller and ends blindly (plate 38, figs. 3 to 9), 
while the other extends into the leaf-blade. In this there is a striking 
similarity between the earlier foliage leaves and the cotyledons, constitut- 
ing a morphological argument against the theory that the cotyledons are 
not primitively leaves. The absence of medullary^ stereome, mentioned 
above in the paragraphs dealing with the hypocotyl, will be noted, and 
this condition, as in the case of retonos, persists until the level of the 
tenth intemode or thereabout. Sections of field seedlings with short inter- 
nodes at a distance of several millimeters from the insertion of the coty- 
ledons show no medullary stereome, and this is true also of medullary 
canals. 

The same section (plate 30, figs. 3, 4) serves, in addition, to show 
very clearly the origin of the periderm, which in the definitive stem, as in 
the earlier intemodes, occurs in the first or outermost cortical layer of cells 
(as shown by Ross, 1908). Fron and Frangois (1901) state differently, 
and their drawing depicts the earliest suberogenous divisions in the 
second layer of cells; in this, however, they are in error. Their drawing is 
taken from a section through the base of a petiole, as the position of the 
leaf-traces, so labeled, shows. In such a section, it is true, the earlier 
divisions will be seen in the second, third, or even fourth layer. I have 
introduced two figures taken from portions of the tissue in question on 

' I use this in a purely descriptive sense. " PerimeduUary stereome" has been 
used. The origin of- this stereome is dealt with beyond (p. no). 



106 Guayule. 

opposite sides of the same section. In the position opposite the first leaf- 
trace the divisions occur in the second layer; at the other end of the 
diameter, in the outermost. The periderm figured by Fron and Francois 
is therefore the leaf absciss layer. Leaf fall in the guayule is consum- 
mated only slowly, and, as compared with more familiar examples of the 
temperate regions, is imperfect in its time relations. The layer is not 
sharply defined, and the disintegration of the tissue is irregular, the result 
of the uneven and irregular character of the component cells of the 
absciss layer. 

The epidermis, both of stem and leaves, in the epicotyl is clothed 
with a single type of trichome found throughout (plate 30, figs. 5 to 11). 
There are two derived kinds, a T-shaped hair predominating, with a few 
scattered hairs of a type seen in Chrysoma (Lloyd, 1901) and in other 
Compositas, viz, the whip-hair, but in which, in the guayule, the terminal 
cell remains undeveloped. The trichome does not, therefore, become 
flagellate, as in the related species, the mariola {Parthenium incanum) , and 
in many Compositse (Vesque, 1885). ^^ certain places, as in the axils of 
the leaves, floral bracts, and corolla, transition forms may be met with, 
indicating that the two kinds have been derived phylogenetically from a 
single type. The fact that both are present in different Compositae, but 
in different ratios, may be used to support the view that the trichome 
clothing is a character which has been brought about by gradual change 
and not by the sudden dropping out of one or the other kind. The 
T-shaped hairs clothe the plant very completely and smoothly, the termi- 
nal cells all lying very nearly parallel to each other, and to the axis, on 
the various organs. The density of the covering varies, however, with the 
size of the organ, as the individual hairs show no substantial amount of 
response to varying external conditions.' 

Before leaving this part of the subject it is necessary to point out 
that in seedHngs in which the stem elongates slowly, as in the field, the 
primary cortical canals of the hypocotyl behave in a manner which has 
not been observed in etiolated plants. The two pairs, associated with the 
median leaf-traces of the two early foliage leaves, instead of passing directly 
into the petioles, anastomose and then, from the transverse lacuna thus 
formed, canals pass off to enter the leaves. Other canals have been noted 
to rise from the lacuna and to pass up into the epicotyl ; a reanastomosis 
within a short distance has been observed (plate 36, figs. 7,8). A section 
of a field seedling made through the cotyledonary node, or at any level, 
if the intemodes are undeveloped, will almost invariably show widely 
spreading divarication of one or more of the canals (plate 36, fig. 6). In 
a word, the canals constitute a branching system, each more or less in 
communication with the other. 

' The mechanical conditions in axils of leaves and in the capitula cause super- 
ficial changes in the shapes of the trichomes. 



Anatomy and Histology. 107 

THE DEFINITIVE STEM. 
PRIMARY STRUCTURE. 

After the tenth internode, approximately, has been laid down, the 
stem takes on its definitive structure. The number and appearance of the 
various structures within the growing tip vary a good deal, according to 
the rate of growth. This is largely due to the crowding together of the 
nodal characters, but in part also to the size of the terminal bud, and 
therefore to the number of leaves. In a thick apex more bundles of 
primary elements appear at a given level (plate 31, fig. 3) ; also the size 
and frequency of branching of the canals is greater within a given zone 
(plate 36, fig. 5). For the purpose of description it will serve to present 
briefly the differences observed, at various levels of a stem one year old, of 
normal growth-rate in the field, as this will give an epitome of the develop- 
ment of the tissues. The specimen before me is a twig which grew in 1 908, 
collected at the close of its elongation for that season. It is 11 cm. long, 
4 mm. in diameter at the base, and 1.6 mm. just behind the tip. The 
structure at the levels mentioned is as follows: 

Within the last millimeter of the tip one finds the vascular tissues 
undifferentiated, though the medulla and vascular zone are recognizable. 
The primary cortical canals appear opposite median leaf-traces,' but 
nowhere else (plate 38, fig. i ; plate 31, fig. 5). The starch sheath (endo- 
dermis) is recognizable only by the starch-content, which appears only 
opposite leaf-traces, while starch is absent from the endodermis else- 
where. Within half a millimeter further down, at a diameter of a milli- 
meter, all the 5 medullary canals appear, 17 vascular bundles are distinct, 
and 1 6 cortical canals are present (plate 3 1 , fig. 4) . In perhaps half of the 
bundles spiral vessels have developed. These are in curved plates of i to 3 
vessels, each separated by wood-parenchyma. The epidermis is densely 
clothed with T-shaped trichomes. The endodermis may be traced com- 
pletely around the stele, on account of its starch. At this level may be seen 
the earliest indications of the stereome bundles in the primary leptome and 
in the pith. 

At 10 mm. from the apex (diameter 2.5 mm.) the collenchyma of 4 to 
6 rows is well developed. The characteristic thickening is first seen in the 
periclinal walls, and these become still more conspicuously thickened 
in the later stages. The larger bundles have xylem plates 6 to 8 cells 
deep radially. Interfascicular cambium is being developed. The stere- 
ome is still thin-walled, but the definitive size of the cells has been reached, 
and thickening has taken place at the angles. In the section before me I 
count 2 5 primary cortical canals and i o medullar}^ canals. The section was 
evidently taken just above the plane in which the pith-canals branched, as 
two of the canals are cut at the fork. 

At 15 mm. (diameter 3 mm.) mechanical elements have appeared in 
the hadrome, and the stereome is more adv'anced as to the thickening of 
the walls. The collenchyma has been somewhat stretched periclinally, 
the walls so placed being much thicker. The walls of both cortical and 

' Very occasionally a pair, a single one on each side of the trace, occurs. 



108 Guayule. 

pith cells have thickened, and in the walls of contact the reticulations, due 
to the broad, ovate, closely-set pits, are very noticeable. The interspaces 
are large. 

At various lower levels, depending on the time of the year in which 
the material is taken, will be encountered the young periderm. Ross' 
speaks of this as beginning very early, and in his material as reaching close 
to the apex. If a newly grown twig is examined toward the close of the 
season it will be found that the periderm embraces only a lower zone (of a 
thickness depending on the rate of growth) at the base of the stem, and its 
growth involves casting off the leaves which remained on the upper por- 
tion of the twig of the previous year. This uppermost zone, carrying the 
overwintered leaves, undergoes some growth with considerable lengthening 
of the internodes, so that the leaf -scars of the winter bud do not crowd 
each other as do the bud-scale scars in plants of the temperate regions. 
The periderm passes upward from this zone, and during the following dry 
season slowly cuts away the leaves, until by midwinter, earlier or later 
according to the character of the season, all the leaves of the previous 
growing season, save the terminal ones, are cast off (plate 14, fig. B). 
As the periderm extends toward the apex of the twig the epidermis is 
fissured concurrently, beginning at the base. 

A section near the base of the season's growth shows the following 
structure: The periderm is three to four cells deep, measuring o.i mm. 
The xylem bundles measure about 0.5 mm. on the radius, and the pith has 
a diameter of i mm. Nearly all the bundles are supplied with both corti- 
cal and medullary stereome. Tracheids are fewer in the outermost zones 
of the xylem. The primary cortical canals and pith canals have generally 
enlarged, the largest measuring 0.3 to 0.4 mm. tangentially, with a radial 
diameter of 0.15 to 0.2 mm. This section has one completed series of sec- 
ondary cortical canals, and a second row begun. The epidermis is slightly 
fissured. This amount of growth and secondary change is by no means the 
maximum possible. The thickest part of the stem of one season's growth 
of the seedling shown in plate 46, fig. A, had five series of secondary 
canals, and cork 0.5 mm. thick, the depth of the cortical tissues, primary 
and secondary, being 2.5 mm. 

A stem of two growth-periods shows the primary and one series of 
secondary canals, but the two seasons' accretions of wood are reflected in 
the annular structure of the wood, as in the seedling hypocotyl before 
mentioned. Here also, therefore, the relation of structure to age is less 
apparent in the cortex than in the wood cylinder. The whole of the outer 
leptome (that embraced between the primary and secondary series of 
canals) , is stereomatic ; that within the secondary series still retains its 
sieve character. A considerable thickness of cork has developed. 

Later changes need not be followed year by year, and it will suffice 
to point out the more important features summarily. The inner periderm 
normally does not begin until the stem attains a diameter of over 10 mm. 
(Ross, /. c), and the primary cortical canals may still be found up to this 
time or even very much later, e.g., in a stem 28 mm. in diameter, with cor- 
tex, including bark, 5 mm. thick. The penetration of the inner periderm 

' His material appears to have been collected in December. 



Anatomy arid Histology. 109 

is not a clean-cut process, such as we see in our common trees and shrubs, 
but first appears directly opposite either a primary stereome bundle or a 
primary canal, as an ingrowth, simulating invagination (plate 32, fig. 4). 
The absciss layer which effects leaf-fall is similarly clumsy, so to speak. 
This tissue consists of a quite irregular layer of cork-cells, continuous with 
the primary phellogen. The outermost cells, those, namely, immediately 
beneath the base of the leaf, first become suberized. 

Until an advanced age is attained, the inner periderm does not cut 
deeply. In old stems, 20 to 50 years of age, light-colored layers of cork 
may be seen penetrating to half the depth of the cortical tissues, but quite 
irregularly. It is of interest to note here that this cork presents a special 
practical difficulty in the factory in handling the comminuted shrub after 
it has passed through the pebble-mill. The bagasse is then, with the 
exception of this cork, which has been broken up into flakes, separated in 
water, the rubber and the cork flakes floating and the remainder sinking. 
Only by means of pressure under water or prolonged soaking may the 
cork be waterlogged, when it sinks, leaving the clean rubber still floating. 
These layers of cork are seen in plate 2, fig. B, from a photograph of a 
stem certainly forty years old. 

SECONDARY STRUCTURE. 

The secondary cortex is characterized by alternating concentric rows 
of stereome bundles and resin-canals. Between succeeding stereome mass 
and leptome parenchyma (canal-cells, consisting of endothelium and usu- 
ally a single subjacent or supporting layer), there frequently intervenes 
no tissue at all, and the stereome occupies the whole of the space between 
adjacent resin-canals. In the inner part of the secondary cortex one finds 
alternating canals and "soft" leptome, the composition of which raises 
some points of question. The canals, as described correctly by Ross 
(1908), arise as a double row of cells derived directly from the cambium 
(plate 22, fig. 13). Surrounding the "secreting" cells is at least one layer 
of leptome parenchyma, the usual condition in slowly growing plants. In 
irrigated plants there may be two or three (or occasionally more) layers 
(plate 29, fig. i). This is followed, radially, by a mass of sieve-tissue 
(plate 32, fig. 2), which may be regular in transverse outline, and com- 
pletely uninterrupted by parenchyma until another canal is laid down, 
or it may be narrow and more or less irregular, as in irrigated plants 
(plate 25, fig. 8; plate 29, fig. 4). In any event, the sieve-tissue occupies 
the radially placed space, broadly speaking, between successive canals, 
and it is in this space that we find stereome later. 

The manner in which the stereome arises is, in broad outline, as 
follows: The outermost (on the radius) leptome cells undergo trans\erse 
enlargement and become stereomatic. Successively other adjacent cells 
lying farther in behave similarly. The resulting tissue, however, occu- 
pies more space than did the original cells from which it arose. As the 
total space which is occupied by the stereome is usually identical with the 
total leptome, it follows that there must be some readjustment. This is 
brought about by the discontinuous sclerosis of the leptome, so that irreg- 
ularly alternating masses of this are destroyed and become compressed. 



110 Gnayule. 

The stereome develops, therefore, within the leptome,^ and in its defin- 
itive form a portion of the leptome comes to occupy the volume of the 
whole. The definitive stereome may be flanked more or less completely 
by sclerosed leptome parenchyma, and even the adjacent cortical cells, 
especially in the peduncle, may take on this character. 

With reference to origin, in general terms, I am at variance with 
Ross, who says on this point: " Durch die Tatigkeit des Kambiums ent- 
stehen abwechselnde Gruppen von zartwandigen Elementen und von Scle- 
renchymfasern. In der jiingsten Gruppe der letzteren geht die Verdick- 
ung der Zellwande erst sehr allmalich vor sich, und in den zartwandigen 
Schichten zwischen dieser und dem Kambium kommt der Secretkanal 
zur Ausbildung." I beheve that I am not unduly criticizing Ross's state- 
ment by saying that it is misleading. It would seem more consonant with 
the facts to say that through the activity of the cambium alternating 
groups of leptome parenchyma and prosenchyma arise, and that the 
stereome arises within the latter. The resin-canals arise from two adja- 
cent tangential layers of the thin-walled parenchyma. 

The change of any particular cells into stereome is not complete 
before the end of the third season's growth, as nearly as we may judge. 
This secondary occupation of the leptome by the stereome is particuarly 
well shown in a preparation made of the cortex of an old stem (plate 32, 
fig. 2). The sections were treated with xylol so as to extract the rubber, 
leaving the tissues empty and distinct. The stereome was seen to occupy, 
with few exceptions, all the space previously occupied by the sieve-tissue. 

ORIGIN OF THE MEDULLARY AND CORTICAL STEREOME. 
Vuillemin, 1884c, p. 223, has described stereome as arising in the 
pericycle in the Compositae, but he does not show its precise origin nor 
that of its constituent elements ; nor does his description of the leptome 
(I.e., p. 99) fit the conditions found in Parthenium argentatum. Accord- 
ing to Vuillemin, the sieve-tubes are of much larger transverse diameter 
than the companion cells, and this is not true of our plant. There are, 
however, broad elements with oblique end-walls,^ intermixed with sieve- 
tubes and companion-cells to form a melange in which the sieve elements 
are generally in contact with each other throughout the whole leptome 
mass, and do not usually form isolated islands, as generally described for 
the Compositce. These elements have common origin in cambium cells; 
that is to say, the broad elements and the narrow sieve-tissue elements are 
of common descent. The broad cells, which later are transformed into 
stereome, do not, therefore, have a distinct origin. The initial division 
within the mother-cell may be periclinal or radial, separating a broad ele- 
ment, destined to become stereomatic, from a similar one, which again 
divides once or twice, usually twice, to form the sieve-tissue (plate 31, 
fig. 9). There is but little difference in the transverse diameter of these, 
the companion-cells being narrowly fusiform and therefore thickest at the 
middle, while the reverse, of course, is true of the sieve-cells. The broad 
elements are recognizable both by their size and by their more tenuous 

' Vuillemin's description, "sur le dos des faisceaux lib^riens," does not apply. 
*The "libriform" of Schwendener, 1874. 



Anatomy and Histology. Ill 

protoplasmic content. When they become stereomatic the first step is the 
great enlargement of their transverse diameters, their walls being thin 
except at the angles, which are thickened after the fashion of collenchyma 
(plate 3 1 , fig. 8) . During this phase of change the mutual pressure of the 
developing stereome and the surrounding cortex results in the radial 
flattening of the latter, and frequently in a crumpHng of the walls in the 
stereome. The limit of the stereome may readily be seen because of the 
intercellular spaces between the cortical cells and those of the stereome. 
Meanwhile the sieve-tubes and companion-cells become displaced and, 
with sclerosis of the stereome elements, are destroyed, and may only with 
difficulty be observed at all. Sclerosis of the stereome proceeds radially 
from without inwardly. The compactness of the stereome, as also its 
regularity and dimensions, depends upon the previous mode of growth of 
the leptome as a whole, and is therefore more irregular and of uneven 
texture, in irrigated plants, or, what amounts to the same thing, in rapidly 
grown plants. Sclerosis also overtakes some of the adjacent leptome 
parenchyma and, under certain circumstances, some of the neighboring 
cortical cells, but is not preceded by their enlargement. 

The stereome in the medulla (plate 3 1 , figs. 6, 7) ,which has previously 
been so referred to for convenience, is, like the above-described leptome- 
stereome, a constituent of the mestome strand. It arises from elongated 
elements clustered about the primary hadrome elements, and is the en- 
doxyle of Briquet (1892), but, in the light of the occurrence of bicollateral 
bundles in the Chicoriaceae (Vuillemin, 1884a; van Tieghem, 1884), may be 
susceptible of another interpretation, viz, that it represents the internal 
leptome in these forms. This explanation is not decreased by the very 
close analogy between the stereome of the leptome and of the hadrome. 
In the young condition the tissue which is destined to become stereome is 
recognizable (plate 31, fig. 6), in transverse section, by the absence of in- 
tercellular spaces and the somewhat thickened angles, which, during the 
stretching of the walls previous to sclerosis, become more apparent, as in 
the case of the leptome-stereome. Interspaces occur in the adjacent pith 
and in the hadrome parenchym. The tissue, taking the form of an irreg- 
ular lunate arc in transverse section, is, therefore, while in contact with 
the hadrome, not to be referred to this without careful consideration. 
The progress of change into stereome is identical in all respects with the 
leptome-stereome, and calls for no particular description; this refers also 
to the mutual displacement of tissues (plate 31, fig. 7). The analogy to 
the leptome-stereome is strengthened by the circumstance that longitu- 
dinal divisions may take place in the earliest formed elements, before the 
final complement of stereome cells is arrived at, though it must be said 
that these divisions are not of sufficiently frequent occurrence to enable one 
to see more than a very few at a time. The form of the elements further 
likens them to the analogous ones in the leptome, being elongated and 
having slightly inclined end-walls. I am therefore inclined to regard the 
medullary stereome as a tissue per se with respect to the hadrome, and as 
having much in common with the stereome of the leptome, so that it 
would seem to be properly regarded as representing the internal leptome 
in genera of the Chicoreaceae 



112 Guayule. 

Precisely these relations occur, to all appearances, in certain of the 
Boraginaceas, e.g., Symphytum tuberosum., Nonnea alba, Omphalodia lini- 
folia, etc. (Jodin, 1902). Concerning the leptome, Jodin says, after speak- 
ing of the disappearance by crushing of the sieve and companion elements 
("les primaires tubes cribles"): 

En meme temps que s'accroissent les ^l^ments liberiens primaires, on peut 

assister, dans certains genres k un ^paississement notable de leurs parois * * * * 

Dans d'autres cas, cet ^paississement est tres faible ou meme n'a pas lieu * * * 
(I.e., p. 308.) 

But no such thickening takes place in the secondary leptome. Appar- 
ently the thickening of which Jodin speaks goes no further. He does not 
trace the precise origin of the cells with thickened walls. 

As to the medullary stereome, he says little, but his figures show very 
clearlv the earlier, prestereomatic condition which I have shown in my 
own figure (plate 31, fig. 6). To quote again: 

Nous aurons peu de choses k dire de la moelle; nous avons eu occasion de 
parler, a propos des faisceaux du bois, de la zone perimMuUaire, et des rayons 
m^duUaires. La region m^duUaire proprement dite se distingue par la taille de 
ses cellules qui sont arrondies en coupe transversale, et qui laissent entre elles de 
nombreux meats triangulaires. (/. c, p. 322.) 

This author, it is seen, points out the same distinctions between the 
perimedullary zone and pith which I have already made. From this com- 
parison between the guayule and the borages it seems clear that we are 
dealing with the same behavior, with the very interesting distinction that 
in the guayule the histological differentiation of the fibers proceeds to 
completion, while in the plants studied by Jodin they are arrested in their 
course of development. This appears to be connected with the herba- 
ceous character of the stems in the Boraginaceae. 

In this connection, Schwendener's observations on certain Composi- 
tas are of particular interest: 

Im Phloem der grSsseren Aster und SoUdago Formen, * * * kommen inner- 
halb der starken primaren Bastbtindel kleine secundare Gruppen mechanischer 
Zellen zur Entwickelung, welche zum Theil mit den kiirzesten Libriformzellen, 
die iiberhaupt vorkommen, ubereinstimmen, und jedenfalls durchgehend vom 
typischen Bast verschieden sind. Die Lange diesen Zellen variert zwischen 150- 
300 Mik. ; die Kiirzesten erreichen oft nur bis 80 Mik. Dazu kommt, dass die 
nebeneinander liegenden schiefen Querwande ahnliche Zick-zacklinien bilden, wie 
sie sonst nur in kurzzelligen Libriform vorzukommen pflegen. Bei Aster bilden sie 
im Querschnitt netzformige anastomosirende Bilden, zwischen denen ein parenchy- 
matisches Cambiform stellenweise mit deutlichen Siebrohren, eingebettet liegt.' 

This distinction made by Schwendener between the sclerosed element 
of the "phloem" and typical bast applies throughout to Parthenium ar- 
gentatum. This plant, however, differs in the distribution of the sclerosed 
elements, forming as they do dense masses occupying the space previously 
occupied by the whole of the leptome and its associated libriform. 

Schwendener, however, appears to assume the independent origin 
of the libriform cells in the leptome, and it is on this point that I advance 
the view that they have a common origin with the sieve and companion 

' Schwendener, 1874, p. 152. I have not had access to this paper. 



Anatomy and Histology. 113 

cells. After arriving at this conclusion, I found that Servettaz (1909, p. 
232) had already done so with respect to certain of the Eleagnaceae. The 
close resemblance in the behavior of the medullary mass of stereome to 
that of the leptome forces criticism of this view to the effect that the 
analogy which I have drawn is based on the origin of the stereome in 
the hadrome and leptome from libriform of an identical mode of origin 
on either side of the cambium. This view, while admittedly possible, does 
not agree with my observations, and it is hoped that further research will 
bring evidence to light which will show which view is correct. 

The secondary resin-canals, when fully formed, are composed of an 
endothelium backed usually by one row of leptome parenchyma (plate 29, 
figs. 1,2). In transverse outline, after full development, they are rounded, 
but gradually become compressed radially as they pass outward toward 
the bark. The youngest ones measure upward of 0.2 mm. in tangential 
diameter, and grow in size till, at the outer part of the living cortex, they 
mav measure, in a cortex 5 mm. thick, over a millimeter tangentially, 
and with a width a third of this. The secreting cells undergo more or less 
periclinal divisions (with reference to the axis of the canal), producing 
sometimes two to three layers of cells of endothelial origin. The resin- 
canals at length frequently become partl>' or completely closed by an 
ingrowth of tissue (Lloyd, 19086) of the same character as the cortex and 
forming an interesting analogy to tracheal plugs (tyloses). These I call 
pseudotyloses (plate 3 2) . The cells of the pseudotyloses at length become 
filled with rubber and continue in a living condition somewhat longer 
than the surrounding cortical tissue, retaining their normal appearance 
when the cortical cells toward the outside of the stem have passed over 
into suber. These parenchymatous plugs are not confined to the very old 
tissues, but may be found also in young stems and roots,' though less 
frequently. Occasionally the medullary canals, in old plants at any rate, 
become partially plugged in the same manner (plate 3 2 , fig. 3) . In addition 
to these outgrowths, resembling roughly a bunch of grapes, one frequently 
finds trichome-like structures, sometimes projecting from the walls and 
also from the plug-tissue (plate 32, figs. 1,6). Somewhat similar appear- 
ances have been observed by Col, and to this I shall call attention again. 
In this connection, however, I feel inclined not to agree with this author 
in his criticism of Vuillemin, who recorded observing structures which he 
called " poils glanduleux " in the canals in old rhizomes of Arnica tnontana 
(Col, 1903, p. 166). I suspect that these "poils glanduleux" are the same 
structures as those which I have called pseudotyloses. 

The pith undergoes a considerable amount of secondary enlarge- 
ment, so that in a stem 2.5 cm. in diameter, in which it may still be found 
in a living condition, its diameter is between 3 and 4 mm. and is irregular 
in outline. The medullary stereome does not receive any secondary accre- 
tion, but the growth of the inner part of the parenchyma rays concomitant 
with that of the pith, between the edges of the xylem wedges and the 
flanking stereome, results in the periclinal separation of these. Sometimes 
one may find that cells near the periphery of the pith have undergone a 



' I have observed them in the primary canals (plate 32, fig. 7). 



114 Guayule. 

rather regularly repeated periclinal division, and the tissue, therefore, has 
much the appearance of a cambium. It may also happen that repeated 
divisions occur in a zone about one of the medullary canals. The cause of 
this is not clear, though it is possible that this also is a mode of growth of 
the pith (plate 42, fig. 5). It does not appear to be the same as the forma- 
tion of cork, such as I have observed to occur following injury to the pith 
or adjacent tissues. 

In field plants normally neither pith nor parenchyma rays (save a 
very few cells) ever become lignified. 

The wood in large stems shows the usual distinction of alburnum and 
duramen. The latter is reddish-brown in color, and all the tracheids are 
plugged by " Gummipropfen. " ' Temme (1885) and Ross (1908) note 
their positive reaction to phloroglucin , which I have verified. They are 
very sharply confined to the duramen in uninjured stems. In one, 2.5 cm. 
in diameter, in which the plugs are beginning to be formed with irregular- 
ity, their genesis may be followed. They first appear as a thin, partial or 
complete lining, increasing irregularly and gradually filling the lumen. 
Their conformation suggests the behavior of a dense fluid. Their positive 
reaction to aniline blue, which is very marked, may indicate that they are 
at first similar to callus, but, as phloroglucin shows, they later become 
lignified. In the old wood the plugs appear homogeneous, but they stain 
unevenly with, e.g., Bismarck brown. Here and there one may note a 
stratification in planes parallel to the surface of the lumen. That resins 
are absent from these structures is shown by their total failure to react to 
alkanet. Molisch (Zimmerman-Humphrey, 1893) showed that gum-plugs 
behave, with certain reagents, like lignified membranes, but a total par- 
allelism is denied by the above reactions. Lignification in any event 
would appear to be a secondary feature. Tschirch (1906, p. 1180) identi- 
fies the substance as "bassorin." 

ANNULAR STRUCTURE. 

The mature wood shows to the naked eye an annular structure which 
is frequently regarded as annual-ring structure. In an old stem what is 
seen in part is a banded appearance due to differences in color intensity 
(plate 2 , fig. B) , having no relation at all to a true annular structure, which 
is readily seen under magnification. This is shown in the two figures, one 
of which (plate t^t^, fig. i) was drawn to scale from the inner alburnum of 
a very old stem, and the other (plate :^2), fig. 2) from one a centimeter in 
diameter, showing ten rings. It is not at all unlikely that these rings 
represent ten years' growth, but this would not justify the conclusion that 
the rings are always correlated with age in years. It must not infrequently 
be the case that more than two accretions of growth occur in response to 
the distribution, in time, of the rainfall, and these rings, therefore, repre- 
sent periods of growth following rain. That these growth-periods for field 
plants usually coincide with the summer seasons follows from the general 

*" Wound-gum " (Temme, 1885) seems hardly a suitable term, since the 
phenomenon is perfectly normal, though, as will appear, the earlier secretion is 
provoked by natural and by artificial wounding. A direct translation of Gummi- 
propfen would be preferable. 



Anatomy and Histology. 



115 



character of the precipitation, as elsewhere described. The evidence gives 
strong support to the view expressed by Holtermann (1907) that the ring- 
structure of the wood is correlated with cessation and resumption of tran- 
spiration. While it is not clear why an annular structure within the annual 
ring is present in the wood of irrigated plants, it is quite possible that 
it is due to stimulation by successive irrigations.* These considerations 
show that it is practically very difficult to determine the age of a plant 
by counting the rings, and this is rendered still more so by their short 
radial measurements. In the case before us (plate ^t,, fig. 2) 10 rings 
are counted on a radius of 2 mm., so that the rings, taken altogether, 
have an average thickness of 0.2 mm. Excluding the innermost (the first 
season's growth) and the deepest, the rings vary from 0.06 to 0.3 mm. 
approximately. This, coupled with their frequently great irregularity and 
indistinctness (Ross, 1908), makes them difficult of recognition. 

The suggestion has been indicated inferentially in this connection by 
Ross that the age of a stem is to be inferred from the number of secondary 
canals and stratifications of the secondary cortex. A stem examined by 
him, 19 mm. in diameter, showed eight zones of canals and of alternating 
phloem layers, and he agrees with Endlich that the stem was about ten 
years old. I feel quite sure, however, that this inference is far from jus- 
tified. For example, the stem from which plate 33, fig. 2, was taken 
was certainly over four years of age, and as certainly eight to ten. There 
were only four rows of secondary canals. In a stem with a radius of i cm. 
I count at least ten cortical zones, while there are but five in another cor- 
tex of the same thickness. These, together with the further fact that 
under irrigation a seedling in five months developed five rows of second- 
ary canals, show that the number of canals depends upon the rate of 
growth and not upon the number of seasons, and in field plants the num- 
ber of rows of canals is, roughly, a third to a half less than the age of the 
stem in years. A cortex 5 mm. thick, exclusive of the cork, taken from a 
very old plant, about forty years of age, shows about twenty canal zones. 
Some had of course been cut out by periderm, but scarcely as many as 
twenty. 

Table 40. — Determinations by weight of ratios of bark to wood infield plants 

(Whittelsey, 1909). 



Plant. 


Parts of the plant examined. 


Ratio 
of bark 
to wood. 


Plant. 


Parts of the plant examined. 


Ratio 
of bark 
to wood. 


I 
II 


Roots and thicker stems 
/ Roots 


''36 
I 0-79 

0.85 
I 13 


II 


Branches and twigs.. . . 


I .29 
1.63 
1.30 

\:l 

1-9 
2 . 1 

1 


\ Trunk 





116 



Guayule. 



THE EFFECT OF ABUNDANT WATER UPON ANATOMICAL 

STRUCTURE.' 

The effect of irrigation upon the structure of the mature plant is very 
marked. This is especially noticeable with respect to the relative volumes 
of the wood cylinder (including pith and medullary rays) and the "bark" 
(cortex and cork). As this is a question of prime importance economic- 
ally, it will be treated first. 

By means of weighing, Whittelsey (1909) determined that, in vari- 
ous portions of the plant, the trunks are made up of 44 to 65 per cent bark 
(cortex and cork) , the amount of bark being relatively larger in the smaller 
twigs. The material was quite dry. (Table 40, page 115.) 

In Whittelsey 's determinations, the pieces examined were first steamed 
to render it possible to separate the wood from the cortex. A slight 



/ 


~^ 




/ -'' 


--^ 


\ 


/ / 




1 FietA 


\ \^ 


/' 


1 


\ N. 


! ^ / 




^^ 


^-^ 





Dry 




Irrip. 



Fio. xfl.— Relative dimensions of wood cylinder and cortex, wet and dry, in twigs of field and 

irrigated plants. X20. 

error is introduced by this method, as some of the resin exudes from the 
cut ends of the cortex and infiltrates into the wood. This error is appar- 
ent in table 41 , in which the ratios of steamed material are smaller than in 

Table 41. — Ratio of bark to wood by weight for field and irrigated shrub, as deter- 
mined after (a) steaming, (b) moist chamber. 

Two pieces each of field and irrigated plants (Cedros, April 1909): (a) Field 
plant pieces 3.9 to 4.8 mm. diameter; (b) irrigated plant pieces, 3.4 to 4 mm. diam- 
eter. Each piece of (a) segmented into fourths and alternate fourths placed in 
each of two lots; (b) segmented into fourteenths, similarly placed in each of two 
lots. One lot (I) of (a) and of (6) steamed, de-barked, dried in oven, and weighed. 
The other lot (II) of each placed in a moist chamber till fit for separating wood and 
bark; then dried in oven and weighed. 



Class of plant. 


Treatment. 


Weight of Weight of 
wood. bark. 


Ratio. 


(a) I. Field (Cedros) 


Steamed 

Moist chamber.. . 

Steamed 

Moist chamber. . . 


gram. 
0. 1604 
0. 1663 
0.311 
0.324 


gram. 
0.2817 
0.2968 
0.318 
0.3608 


1-75 + 
1.78 + 
1 .02 
I . II 


(a) II. Field (Cedros) 

(b) I. Irrig. (Cedros) 


1 (6) II. Irrig. (Cedros) 



* The substance of what follows under this caption was presented in a paper 
entitled "The Responses of the Guayule, Partheniinn argentatmn Gray, to Irriga- 
tion," before the Botanical Society of America, at its Boston meeting, December 
1909. (Lloyd, 19106.) 



Anatomy and Histology. 



117 



control material softened in a moist chamber. But the small error, less 
than I per cent, is of significance only when such data are used in large 
calculations. 

From the present point of view a volumetric method is of more value 
than weighing, since the ratios derived by the latter are disturbed by vari- 
ation in the specific gravity ; but as a comparison 
of the ratios derived by both methods is of use in 
practice, they have both been introduced. For 

similar reasons it is im- 

corTt I portant to know the 

ratios derived from dry 
material, and for this 
purpose the method of 
displacement of alcohol 
has been used. 

RELATIVE VOLUMES 
OF CORTEX AND WOOD. 

The material from 
which the data tabulated 
in table 42 were obtained 
was collected at Cedros. 
The irrigated material 
was taken in Se^"*. ember 1908 
from stems (fig. 16) of two sea- 
sons' growth. The difference in 
thickness of wet and dry cortex 
is very slight, and is not given. 

It is seen that the volume 
ratio of bark to wood (when dry) 
in the irrigated plant is near to 
unity in the smaller twigs to o. 2 7 
in the larger, up to a diameter 
of 13.5 mm., beyond which no 
material was available. In field 
plants the ratio for the smaller twigs approaches 2.5, being reduced to 
1.7 for stems 13 mm. in diameter, and still further, namely, to unity, for 
stems exceeding this diameter (20 mm. and more). From the economic 
point of view this material reduction of cortical tissues in irrigated plants 
is an important consideration, since it is these tissues which contain the 
rubber. 

The ratios for the wet tissues indicate the large water-holding capac- 
ity of the irrigated cortex, especially as compared with the field material. 
These differences in volume are quite obvious in the radial measurements 
of the wood and bark. In tables 43 and 44 some accurately made 
measurements are given. The ratios are illustrated in figs. 15, 16. For 
the better direct comparison of field and irrigated plants dry twigs of the 
same initial total diameter were chosen and were measured both dry and 
after being soaked in water. The initial size is shown in the diagrams 




Fig. 17. — Relative thickness of cortex in stems of irri- 
gated and field plants, the wood cylinders being 
of equal diameter when dry. 



118 



Guayule. 



by a half-circle. To the right of the vertical diameter is shown the irri- 
gated plant; to the left of it the field plant. The upper quadrants show 
these when wet ; the lower, when dry. To be noted are the greater capacity 
of the wood cylinder in field plants for swelling, due to the larger volume 
of the parenchyma rays; and the smaller capacity of the cortex tissues for 
swelling, due to the larger rubber-content. The greater volume of cork 









Tab 


LE 42. — Volume of wood and cortex {Cedros, Sept. 1 


908). 




Irrigated plants. 


Field plants. 


Diameter 


Thick- 


Ratio of cortex 


Diameter 


Thick- 


Ratio of cortex 


of wood 


ness of 


to wood by 


of wood 


ness of 


to wood by 


cylinder.v 


cortex. 


volume. 


cylinder. 


cortex. 


volume. 


Dry. 


Wet. 


Wet. 


Dry. 


Wet. 


Dry. 


Wet. 


Wet. 


Dry. 


Wet. 


mm. 


mm. 


mm. 






mm. 


m-m. 


mm. 






a .0 2.0 


0.7 


0-93 


1.65 


1-3 


1-7 


0.9 


2.33 


2.5 


2 


3 2 


4 


0.8 


0.81 


1-35 


2 


7 


2.9 


1-5 


2 .0 


2 .65 


3 


3 


3 


7 


I.I 


0.94 


1.28 


3 


7 


4-3 


1-7 


1-5 


2 . I 


4 


b 


4 


8 


I . I 


0.565 


1.23 


4 


I 


4.7 • 


2 .0 


1-5 


2.25 


7 





7 


.=; 


1-4 


0.43 


0.71 


5 


9 


6.0 


2.8 


1-7 


2 . I 






13 





2 .0 


0.33 


0.6 


6 


2 


6.6 


3-0 


1-7 


2.08 






13 


5 


2 .0 


0.27 


0-55 






14.0 
22 .0 
22 . 7 


3-5 
5-0 
4.9 




I . II 
I . II 

1-05 



and of the cortical intercellular spaces in irrigated plants must also be 
considered. As these tissues are included in the tables under the term 
" bark," it is obvious that an error is introduced which is larger for the irri- 
gated plants. Hence the ratios ought to be, for these, relatively smaller. 
Table 44 shows the same relations for branches of larger size, in which 
the ratios of bark to wood are smaller, but relatively more so in irrigated 
plants. The figures are of special interest, as they include the ratio seen 

Table 43. — Transverse dimensions of terminal twigs of irrigated and field plants of 
the same initial size, before and after drying (fig. 15). 





Total 
diameter. 


Diameter 
of wood. 


Thickness of 
cortex. 


Thickness of 
cork. 


Ratio of field 

to irrigated 

cortex, volume. 


Dry. 


Wet. 


Dry. 


Wet. 


Dry. 


Wet. 


Dry. 


Wet. 


Dry. 


Wet. 


(a) Irrigated. . 
(6) Field 

(c) Irrigated. . 

(d) Field 


mm. 

2.6 

2.6 
3-9 

3-9 


mm. 
3-45 
3-15 
4-85 

4-5 


mm. 
1.85 

1-5 
30 

2-3 


mm. 
2.0 

1-7 
3-15 

2-5 


mm. 

0-375 
0-55 
0.45 
0.8 


mm. 
0.725 
0.725 
0.925 

I.O 


mm. 
0.13 

O.IO 

o.is 

O.IO 


mm. 
0.16 

O.10-O.15 
0.19 
0.11-O.15 


J 2.12 


0-9S 
1. 17 



in a plant from the Hacienda de San Isidro, near Escalon, Chihuahua, 
where guayule is said to grow rapidly. While the rate of growth is not as 
great as supposed, nevertheless it is sufficiently so to be reflected in the 
structure of the stem, which is intermediate in character between Cedros 
field and irrigated plants. In the three cases the initial wood cylinder 
diameter (20 mm.) was the same — in this way the largest available sizes 
could be compared. The thickness of the cork is, when wet, nearly the 



Anatomy and Histology. 



119 



same in all, though its irregularity makes accurate measurement impos- 
sible. When dry it is thickest in the Cedros field plant, thinnest in the 
Chihuahuan plant, and intermediate (though much more irregular in 
thickness) in the Cedros irrigated plant. The differences in the cortex are 

Table 44. — Comparative radial measurements, in millimeters, of medium-sized stems 
of guaytile, wet and dry. Wood {dry) cylinder 20 mm. diam. in all. August 
29, 1909 {fig. 16). 



Kind of plant. 


Dry. 


Wet. 


Ratio of 

cortex 

to wood, 

by 
volume. 


Wood. 


Bark 
(cortex 
+cork). 


Cortex 

(without 

cork). 


Wood. 


Bark 
(cortex 
+ cork). 


Cortex 

(without 

cork). 


Cedros irrigated, 3 
years old 

San Isidro (near 
Escalon) field. . . 

Cedros field 


mm. 

10 

10 
10 


mm. 
1-7 

2.5 

4.35 


mm. 
1-3 
2.3s 

3-75 


mm,. 

10.25 

10.25 
IO-37 


mm. 
2.5 

3-4 
5-25 


mm. 
2 .0 

2-95 
4.8 


0.28 

0-53 
0.89 



The irregularity in the thickness of the cork makes it difficult to measure it 
properly. It is, at all events, less than i mm., and relatively thinner in field plants. 

apparent ; the index of imbibition of the wood cylinder, while still greater 
in the Cedros field plants, is relatively much smaller than in smaller stems, 
because of the compression of the medullary rays. 

Tables 45 and 46 are based upon comparative measurements of Cedros 
field and irrigated plants. The latter material, however, was collected in 

Table 45. — Relative amount of bark and wood in guayule, by volume {dry). Irrigated 

plants {Cedros, Apr. 1909). 



No. 


Total 
diameter. 


Diameter 
of wood. 


Thickness 
of bark. 


Ratio of 

volume of bark 

to wood. 




mm. 


mm. 


mm. 




I 


ISO 


13.0 


I .0 


0.32 


2 


9. 12 


732 


0.9 


0.48 


3 


8.3 


6.7 


0.8 


0. 50 


4 


5.8 


4.25 


0.77 


0.79 


5 


5.6 


4.35 


0.62 


0.70 


6 


5.22 


3-75 


0.735 


I .00 


7 


3-5 


2.6 


0.4S 


I .00 


8 


2.4 


1-4 


O-S 


I .46 


9 


2.15 


1-4 


0.37 


I . II 


10 


1-52 


0-95 


0.29 


1.67 


II 


21.7 


18.6 


1-55 


0.31 


12 


12.4 


10.9 


0.75 


0.40 



Nos. I to 10, stems from single plant taken Apr. 1909. No. 11, stem from 
different plant taken Sept. 1908. This is the trunk (main) brought from Saltillo. 
No. 12, root from plant taken Sept. 1908. No. 8, base of piece through crowded 
nodes, bottom of 1909 growth, hence bark a little thicker here. 

April 1909. At this time growth had only just recommenced, from which it 
is evident that the amount of water received between September 1908 
(at which time I left Cedros) and the time of my later visit was very 
small — and the information obtained showed this to have been the case. 
The chief value of these tables, besides indicating somewhat more 
fully the points already made, lies in the evidence they bear that the ratio 



120 



Guayule. 



of bark to wood has increased during the period between the dates given 
above, as shown in brief in table 47. 

As will be seen, this change in volume in the irrigated cortex is to be 
referred chiefly to an increase in the rubber-content. 

Table 46. — Relative amount of bark and wood in guayule, by volume {dry). Field 

plants (Sept. 1908). 



No. 


Total 
diameter. 


Diameter 
of wood. 


Thick- 
ness of 
bark. 


Ratio of 

vol. of bark 

to wood. 


No. 


Total 
diameter. 


Diameter 
of wood. 


Thick- 
ness of 
bark. 


Ratio of 

vol. of bark 

to wood. 




mtn. 


mm. 


mm. 






mm. 


mm. 


mm. 




I 


29.25 


20.25 


4.5 


I . 11 


11 


4.67 


2.7 


0.99 


2 .0 


2 


9.25 


7 


45 


0.9 


0.57 


12 


4.25 


2.65 


0.8 


1-7 


3 


9.0 


t, 


7 


I. 15 


0.81 


13 


4.2 


2.75 


0. 72 


1.25 


4 


15-45 


ro 


5 


2.47 


I. II 


14 


4.0 


2-5 


0-75 


1-47 


5 


925 


5 


93 


1.7 


1-37 ' 


15 


3-5 


2.45 


0.52 


I-3I 


6 


7-85 


5 





1.4 


1.44 


16 


3-35 


2.25 


0.55 


1-4 


7 


5.8 


3 


9 


0-95 


1.34 


17 


2.6 


1-5 


0-55 


1.25 


8 


5-25 


3 


3 


0.97 


1-45 


18 


1-9 


1.42 


0.24 


1 .00 


9 


5-15 


3 


15 


1 .0 


1-57 , 


19 


24 . 2 


19.0 


2.6 


0.63 


10 


4.9 


3 


12 


0.89 


1.63 j 


20 


IO-75 


7-7 


1-5 


0.80 



No. I, basal portion of trunk; No. 2, a lateral root; No. 3, a tap-root; Nos. 4 
to 18, series from a single plant; No. 19, San Isidro, Chihuahua, large plant; No. 20, 
Chihuahua, trunk, seedling. 

A discrepancy which always appears between the ratios by weight, 
not here given, and by volume is greater for the irrigated plants on ac- 
count of the larger rubber-content of the wood cylinder in the field plants. 

The smaller ratios for the smaller twigs (Nos. 12 to 18 inclusive) for 
field plants in table 46 as compared with those in table 42 are due in 
part to the fact that the material was collected in the height of the grow- 
ing-season, and hence the new growths have something of the character 
of the irrigated plant in its low rubber-content especially. The impor- 
tance of this fact in its relation to practice is shown elsewhere. 

Table 47. — Ratios of bark to wood in Cedros irrigated plants collected in 
September 1908 and in April 1909. 



T-, ^. Range in size of stem 
"^^^- (dry) diameter. 


Limits of ratios for 
these sizes. 


Sept. 1908 (table 42) 

Apr. 1909 (table 45) 


mm. 
3.4 to 9.8 
3 • 5 to 5 . 8 
3-5 to 8.3 


0.93 to 0.43 
I .00 to 0.79 
I .00 to 0.5 



It is interesting to note, further, that the ratios for root-tissues are 
similar in both types of plant, but are smaller in irrigated plants. They 
are also much smaller than those for the stem in each type. This fact 
should be considered in making up averages to indicate the relative eco- 
nomic value of cultivated guayule. The roots, in proper practice, should 
not enter into a calculation for returns in manufacture, and by excluding 
them the average ratios (those, namely, for stems only) are higher, but 
relatively higher for field plants, as shown in the tables above. 



Anatomy and Histology. 121 

EFFECT OF VARIOUS AMOUNTS OF WATER OF IRRIGATION. 

The most fundamental economic question for which an answer will be 
sought in these pages is that relating to the production of rubber by plants 
under irrigation. As bearing upon the answer is the relation of the above 
tissue-responses to the amount of water supplied, as already indicated in 
table 44. That the inference based upon the data there displayed is cor- 
rect is indicated by the measurements taken from irrigated plants from 
two localities where the conditions were unavoidably and markedly dif- 
ferent, as follows: 

Cedros. — Stocks planted March 1907, by Mr. C. T. Andrews. Irri- 
gated freely till April 1908. Went dry till summer rains. 25 cm. growth 
during 1907 and during 1908. Long drought from September 1908 to 
May 1909, but probably irrigated somewhat during this period. Sample 
plant collected May 10, 1909 (plate 17, fig. B). 

Caopas. — Whole plants of medium size planted January 1908, and 
abundantly irrigated till June 1908. On account of failure to start in 
January they were trimmed back down to the stouter branches. New 
shoots then started, these being for the chief part included above under 
"total diameter 3 mm. or less." The Caopas presa broke out in June 
1908, so that between that date and the time of collection (May 9, 1909) 
no irrigation was possible (plate 46, fig B). 

T.-vBLE 48. — Comparison of ratios of bark to wood by weight for plants from 
Cedros and Caopas, irrigated. 



Parts. 



Stems, 3.0 mm. diameter 

7 to 10 mm. diameter 

Larger, up to 23 mm. diameter (wood cylinder) 



Cedros. 



I. 16 
0.89 
0-525 



Caopas. 



1-56 
1 . 20 
0.84 



From the figures it may fairly be concluded that the amount of dis- 
turbance in rate of gro\\^h in the tissues considered is, within certain wide 
limits, related to the amount of available soil-water. The less the water, 
the thicker the bark (cortex) , and vice versa. The Caopas plants certainly 
had less water than the Cedros plants, and the ratios of bark to wood 
stand in these at 1.56 and 1.16, respectively, for the small branches which 
grew in both plants under irrigation. As to the reduction in radial meas- 
urement of the chief rubber-bearing tissue, the cortex, it must be remem- 
bered that this is compensated for by the more rapid growth of plants 
under irrigation (up to six times) , so that the absolute amount of cortical 
tissue in an irrigated plant will be greater than that in a field plant for the 
same period of growth. 

The rate of growth determines the total volume. In order to obtain 
an empirical factor for the purpose of conveying to the mind an approxi- 
mate notion of the relative ability of field and irrigated plants to produce 
" bark " in a given period of time, I took two average twigs of one season's 
growth, removed the leaves, decorticated, and weighted. The figures in 
table 49 were obtained. Here it is evident that, aside from the much 
more rapid growth in weight in irrigated plants, the amount of rubber- 



122 



Guayule. 



Source. 


Weight 
of cortex. 


Ratio of 

cortex of 

irrigated to 

field plant. 


Irrigated plant. . 
Field plant 


grams. 

5-44 

0.97 


}5.a 



bearing tissue formed by a single twig is at least 5.5 times that produced 
on a field twig of similar age. 

By introducing this factor, and that of rapid growth, into the calcula- 
tion, it may be seen that the resulting total volume of the rubber-bearing 

tissue preponderates in the most 
Table 49. rapidly grown plants, and from the 

data set forth there emerges the 
conclusion that it is possible to reg- 
ulate irrigation, and thereby to pre- 
determine, within the usual limits 
approximately shown in the preced- 
ing pages, the relative total amount 
of cortex and wood. I do not forget 
that difficulties of another sort, 
related to the manufacture of crude rubber, may be introduced, but with 
these we are not at present concerned. The remaining part of the ques- 
tion, as to the amount of rubber the tissues of irrigated plants are capable 
of producing, is in part answered beyond, in a succeeding chapter. 

EFFECT OF DROUGHT FOLLOWING IRRIGATION. 

From the ecological point of view, it seems reasonable to argue that 
the greater production of parenchymatous tissues is in the direction of suc- 
culency, and is an adaptive response to the arid conditions under which 
the plant lives. The largest growth of these tissues is found in the paren- 
chyma rays as well as in the cortex, and there can be little doubt that the 
parenchyma of the pith, parenchyma rays, and cortex function to some 
degree of efficiency as water-storage reservoirs. It is, however, clear from 
the measurements which have been presented in the foregoing tables that 
the way in which this succulency works is not by capacity for a large 
amount of water— irrigated plants are superior in this respect — but, it 
must be argued, in holding it more tenaciously. The efficiency in this 
direction is, however, not very great, if we measure it crudely, as when we 
observe the rate of wilting when the plant is removed from the ground, 
and it is not in any sense to be compared with the resistance of desert suc- 
culents in this regard. What the rubber itself may contribute to this mod- 
erate efficiency can be answered only in speculative fashion. The death 
of large numbers of plants scattered over large areas after severe drought 
does not warrant extravagant notions, at any rate. 



EFFECT OF IRRIGATION ON THE PHYSICAL CHARACTERS 

THE WOOD. 



OF 



The wood of irrigated plants is noticeably harder and more rigid than 
that of field plants (Lloyd, 1 9086) . This is apparent upon cutting or upon 
twisting or bending. For the purpose of measuring the differences in flex- 
ural rigidity, two slender wood cylinders of equal (2 mm.) diameter were 
obtained by freeing them from the cortical tissues, and were then sub- 
jected to bending before and after drying. It was found that, when still 
wet, the wood of the irrigated plant is three times more rigid than that of a 
field plant (the exact ratio was 11 to 3.5) and when dry the ratio is 2 to i. 



Anatomy and Histology. 123 

This mechanical difference appears to be due to the nature and 
extent of the medullary rays and their relation to the wood, together with 
the relative amount of mechanical tissue in the latter. The very great 
difference in the size of the parenchyma rays is seen in both transverse 
and tangential sections, as shown in the figures (plate 28, figs. 1,2; plate 
^Ti, figs. 3 to 6), in which it is seen that the rays in field plants are very 
much larger than in irrigated plants. For this reason alone we must con- 
clude, other things equal, that the former would be much the less rigid. 
Further, the walls of the medullary-ray cells in irrigated plants become 
much thickened and lignified (plate ^S' fig- 7). while in field plants the 
cells remain thin-walled indefinitely, with the exception that there occur 
among them a very few tracheid-like cells (plate ^^, fig. 8), with very 
peculiarly thickened walls. These are so few in number that it is difficult 
to attach any physiological or mechanical importance to them. The 
mechanical elements of the wood in the irrigated plants appear more 
compact to the eye, the lacunas being smaller and the whole mass being 
made up of smaller and more regularly developed cells. It may here be 
remarked, also, that the development of medullary stereome is somewhat 
stronger, but this scarcely contributes a measurable quantity to the total 
rigidity of a stem more than i to 2 mm. in diameter. 

The vessels of irrigated wood are frequently plugged with the so- 
called " Gummipropfen " at an age of two years or even less. Their 
appearance is hastened by artificial or by natural wounding, as the dying 
back of the peduncle. The pith-cells may undergo a considerable amount 
of sclerosis, without change of shape. The lumen is frequently very much 
reduced in size, and the walls are traversed by delicate branching canali- 
culi (plate 29, figs. 5,6). Sclerosis of pith-cells occurs in Manihot glaziovii 
near the leaf-bases, that is, at the nodes (Calvert and Boodle, 1887), in the 
pith of Liriodendron tulipijera (Holm, 1909a), and in that of Corniis fior- 
ida (Holm, 19096). The sclerosed cells of the last-named are identical in 
structure with those of the guayule, both as regards the pores and the 
small size of the lumen. Jodin (1902) also notes a total sclerosis of the 
pith in Cynoglossum officinale, and partial in Lithospermum, fruticosum. 
The sclerosis of pith-cells under irrigation suggests the value of experimen- 
tation with other plants in the reverse direction. 



124 Guayule. 



THE PEDUNCLE. 

It has been said elsewhere that the method of branching is correlated 
with the production of the inflorescences, the terminal new branches in 
twos or threes being produced by the outgrowth of the uppermost axillarv 
buds below the peduncle. There is but little secondary thickening in the 
peduncle, while, correlated with its slender character, there is a large de- 
velopment of mechanical tissue (plate 34, fig. i). In the young condition 
the chief points of difference between a peduncle and a definitive foliage 
stem are the absence of medullary canals^ and the interruption of the 
hypodermis by the development of chlorenchyma, without, however, the 
reduction of the collenchymatic character. There are about 6 to 8 of 
these longitudinal chlorophyll strips, provided with numerous stomata 
with their axes placed longitudinally, as is usual. The cortex, which is, of 
course, primary, has usually 6 resin-canals above to 10 below. There are 
about a dozen bundles, and these, before secondary thickening is con- 
cluded, produce a few tracheids, though tracheae are equally prominent 
constituents of the hadrome. A weak interfascicular cambium is formed, 
but its cells, without losing their cambial character, undergo sclerosis, pre- 
serving their rectangular transverse section. Stereome strands are formed , 
as in the stem, just within the pericycle and in the pith, but their relative 
amount of development is here much greater. It spreads toward the 
interfascicular cambium, involving the parenchyma-ray cells, sometimes 
entirely. The stereome within the cambium thus unites to form a complete 
stereomatic sheath, or perhaps is occasionally interrupted by incomplete 
sclerosis of parenchyma rays. Outside of the cambium, the sheath is 
interrupted by the cortical rays, inasmuch as the cells here do not become 
sclerotic, except a few adjacent to the leptome-stereome. The non-scle- 
rosis of the interfascicular cortex comports with the view of the chiefly 
leptomatic origin of the stereome. The small amount of cortical stereome 
adjacent to the leptome-stereome may readily be recognized both by the 
color and shape of the cells. 

In addition to the normal stereome, as this may be called, some of the 
pith-cells, a few of the outer cortical cells, and some also from the collen- 
chyma become stereomatic. The chief part of this adjunctive stereome is 
derived from the pith, which contributes a notable amount to the inner 
surface of the mechanical sheath. In the basal part of the peduncle a 
periderm occurs, giving rise to a layer of cork about o.i mm. thick. No 
absciss tissue is formed, and the dead peduncle persists for years until 
disintegration finally overtakes it. As death extends toward the base of 
the peduncle, the vessels both of the peduncular wood and that of the 
adjacent stem become plugged, as elsewhere described. It is a matter of 
interest to note that the structure of the peduncle is very similar to that of 
the stem in the mariola {Parthenium incanuni) , and bears notable resem- 
blances also to the herbaceous stems of P. hysterophorus and P. lyratum. 

' I have seen one canal in the pith on one occasion. 



Anatomy and Histology. 125 

THE LEAF. 
COTYLEDONS. 

The cotyledons of the guayule are dorsiventral (plate 34, figs. 7 to 9). 
Both surfaces are free from trichomes. The parenchyma is composed of 
six layers of cells, of which the upper two form a palisade tissue. The 
spongy parenchyma is not highly differentiated, for the two lower layers 
of cells only have distinctive characters, and these are not pronounced.* 
There are no resin-canals in the blade, though the four primary canals of 
the hypocotyl pass, two into each of the petioles. The mid-vein (plate 
31, fig. i) in the petiole is composed of two mestome strands, the origin of 
which has already been discussed, but is single above, and there are two 
lateral veins. The cotyledons show certain well-marked responses to 
water and light conditions. The cotyledons of seedlings grown in the 
shade are slightly thinner than those of field seedlings, and have a much 
larger superficies (plate 34, figs. 6, 9). All the parenchyma cells, and all 
the epidermal cells save the guard-cells, are expanded in directions parallel 
to the surface. The epidermal cells of the surface are deeper also. The 
intercellular spaces of the spongy parenchyma are more extensive, and 
the cell-walls are thinner. These changes are in accord with observations 
in general, but it is of importance to note that the internal and external 
adjustments of the cotyledon are produced by changes in shape of the 
cells, and not by change in number of cells. This is well illustrated by 
the behavior of the epidermis, both as to the shape of the cells and the 
number of stomata. One might well suppose that there would be an 
increase in the number of stomata, as well as in their size, in plants well 
supplied with water and not subjected to severe aerial conditions. Counts 
of the stomata per unit of surface gave the results shown in table 50. 

That is, the number of stomata per unit of area appears to depend on 
the amount of growth of the leaf. The greater number on the lower 
surface in the field plant is due to the rolled-leaf effect, which is absent 
from the shade plant. The result is that, in the plant which has to con- 
serve water, there are relatively more stomata to carry it off .^ Evidently 
therefore, the supposedly adaptive adjust- 
ments as regards the stomata do not involve ^°' 

their numbers in plants of the same species 
under different conditions. The thinner and 
less strongly cuticularized epidermis of the 
shade plant may indeed compensate, as may 
also the more extensive intercellular system, 
for the relatively fewer stomata. But inasmuch as the dampering of trans- 
piration by stomata is not effective within wide Hmits (Lloyd, 1908a) , such 
differences in numbers as the above may be of little or no significance.^ 

' The structure is very similar to that of the cotyledon of Helianthtis, in 
which, however, the intercellular spaces are relatively more extensive, and there 
are more layers of cells. 

' Transpiration rate is greater per unit of surface in sun plants than in shade 
plants (Bergen, igoS; Sampson & Allen, 1909). 

3 On this question the student should consult Renner, 1910. 



Surface. 


Shade. 


Field. 


Upper 

Lower .... 


75 

60 


100 
130 



126 Guayule. 

That the structural adjustments of the cotyledon involve only change 
in shape of the cells is shown also by the responses of seedlings grown in 
a soil of high osmotic equivalent (plate 34, figs. 5, 8). Under this condi- 
tion the surface of the cotyledon is greater than in the field plant (plate 34, 
figs. 4, 7), but its thickness is also much greater. The cells of the paren- 
chyma are correspondingly deeper (plate 34, fig. 8), their extension of size 
being at right angles to the leaf-surface and parallel to the direction of 
greater extension of its size. This cotyledon represents the maximum 
response in a xerophytic direction, and it is worthy of note that under 
normal field conditions this response does not ensue, indicating that 
succulency here is primarily the efifect of a soil condition, namely, the low 
physiological availability of the soil-water. 

PROPHYLLS. 

The earliest foliage leaves (prophylls) show a slight advance toward 
the bifacial condition, though normally they are dorsi ventral. Neverthe- 
less it is possible to induce a marked bifacial condition by growing seed- 
lings in soil which contains a very meager supply of water (plate 35, figs. 5, 
8). Such plants grow very slowly indeed, and the earliest foliage leaves 
attain but a small size and are relatively thick, while the resin-canals are 
of greater diameter. The extreme departure from this condition is shown 
by shade-grown leaves (plate 35, figs, i, 2), with the greater superficial ex- 
tent of which the shape and dimensions of their cells are correlated, while 
field seedlings and those grown in soil of high osmotic equivalent are very 
similar in structure. To be noted, however, both in these leaves and in 
the cotyledons, is the behavior of the spongy parenchyma. The lower- 
most layer of cells shows this especially. In shade plants (plate 35, fig. i) 
the cells are broad, as viewed in a transverse section, and dumbbell- 
shaped. In the field plant (plate 35, figs. 3, 4) they are columnar and 
have two spaces between each two cells. In the seedlings exposed to dis- 
tinctly unfavorable soil these spaces are almost, or frequently entirely, 
absent (plate 35, figs. 6, 7). 

THE DEFINITIVE LEAF. 

Although the foliage leaves exhibit a structural advance over the 
cotyledons, it is noticeable that, as compared with these, the definitive 
foliage leaves exhibit a smaller range of response. These have, to be sure, 
a dense clothing of trichomes, described elsewhere, and this fact may 
explain the difference, which receives no elucidation in the character of 
the stomata (plate 35, fig. 11). These show no special so-called adaptive 
features. There can be no doubt that closely packed hairs form an effective 
insulation which has the effect of producing mesophytic conditions, so to 
speak, over the leaf-surface, both by dampering transpiration and by 
modifying the sunlight.^ 

To determine the extremes of structural response within the leaf, I 
have taken one leaf from an irrigated plant during the period of rapid 
growth and one from a large seedling of good size after a six months' 

' C/. Wiegand's interesting paper of 1910. 



Anatomy and Histology. 127 

drought, as, presumably, final resultants of a complete antithesis of soil- 
water conditions. In these one observes a thinner leaf, though slightly 
more strongly cutinized, in the field plant (plate 35, fig. 13), otherwise but 
little difference is to be seen. In both the structure is strongly isolateral, 
with six layers of cells in each, but one may detect a somewhat more exten- 
sive system of intercellular spaces in the irrigated plant (plate 35, fig. 15), 
though it must be said that the difference appears but slight. The canals 
show no appreciable difference. Neither the stomata nor the substoma- 
tal spaces afford any ground for special comment, while the outer epider- 
mal walls are, contrary to expectation, slightly thicker in irrigated plants. 
A denser trichome covering in the field plant may indeed compensate for 
this, but the observed differences are very slight. 

It remains possible that differences in the character of the soil in 
which these plants grew are responsible for the close similarity, but the 
rate of growth in the irrigated plants and their resulting general mesophy- 
tic character minimize the value of the supposition. 



128 



Guayule. 



Abbreviations Used in Plates 22 to 39. 



pc. pericambium. 

end. endodermis. 

ex. exodermis. 

/i,/,. primary and secondary leptome. 

fei./tj. primary and secondary hadrome. 

s. stereome. 

m.c.t. median cotyledonary trace. 

l.c.t. lateral cotyledonary trace. 

c.t. cauline traces. 

c.t.l. cauline traces, first primordial leaf. 



cot. 


cotyledon. 








cor. 


cortex. 








r.c. 


resin-canal. 








ped. peduncle. 








pr. 


parenchyma 


-ray 


(or cells 


of this) 


lib. 


libriform. 








I. 


lacuna. 








ip. 


leptome. 








pd. 


periderm. 









Description of Plate 22. 

1-5. Development of endodermal canals of root. Fig. 3, root 8 mm. diameter. 

6. Lateral grov^th of endodermal cells and intercalated walls, with Caspary's 

band. Root 0.6 mm. in diameter. 

7. Cortex (with exodermis) of a tap-root 0.46 mm. in diameter. First peri- 

dermal divisions in the hypodermal cells. 

8. The reduction of cortical cells into cork has reached the endodermis. 

9. Portion of a tap-root showing early cambium divisions. 

10. The same in a somewhat older root to show development of intercalated 

mestome strand (h^, Q at outer edge of primary hadrome plate (hj). 

11. A still later stage showing closure of secondary hadrome about the parenchyma 

island by uniting with intercalated mestome strands. The separation of 
these from the primary plate is due to disturbance by the growth of a second- 
ary root. 

12. Primary leptome in contact with the endodermis, in the hypocotyl. 

13. A secondary cortical canal arising in the phloem. The meatus is just at this 

moment appearing. 

14. Details of secondary leptome, showing the relation of the resin-canal to the 

remaining elements. Root 4 mm. in diameter. 

15. Pores in walls of endodermal and cortical cells (c/. plate 41, fig. 2). 

16. Pericambium in a root after primary stereome has begun to develop. 



LLOYD 



PLATE 22 




130 Guaynle. 



Description of Plate 23. 

-7. Tap-root. 

1. Field seedling. 

2. Irrigated seedling. 

-5. Field seedling, 2 mm. diameter {cf. plate 40, figs. 2,3). 

3. Sector to show arrangement of tissues and distribution of rubber. 

4. Endodermis and pericambium of a root 1.2 mm. in diameter. 

5. Same, thickened and compressed; root 1.5 mm. in diameter; globules of rubber. 

6. Region just within primary canals. Primary leptome-stereome (5,) ; secondary 

leptome-stereome {s.,)- 

7. Parenchyma ray enlarged to show globules of rubber. 

8. Epidermis of the hypocotyl, first peridermal divisions. 

9. Trichome from hypocotyl. 



LLOYD 



PLATE 23 




132 Gtiayule. 



Description OF Plate 24. 

I. Cotyledon en face, to show distribution of vascular strands. 
2-5. Sections through cotyledonary collar and upper part of hypocotyl in an 
ascending series. 

6. Section through cotyledonary collar, and lower part of first internode. 

7. Section through first node of epicotyl. 

8. Section through stem near second node of epicotyl. 

9-1 1. Section through different levels to show primitive trachea and its relations 
to older elements. 

9. Base of hypocotyl. 

10. Middle of hypocotyl. 

11. Base of cotyledonary collar. 

12. Origin of cauline strands at a level between those of figs. 4 and 5, but in a 

younger specimen. 

13. Diagram of the vascular tissue (hadrome) in the plantlet. Arrows indicate 

fusion; dotted line the primary trachea. 



LLOYD 



PLATE 24 



CTV 




134 Gnayule. 



Description of Plate 25. 

I --6. Primary endodermal resin-canals of the hypocotyl. 

1. Cell lineage (diagrammatic). 

2. A not infrequent but abnormal behavior. 

3. Definitive condition, as regards cell-walls. 

4. Definitive condition after growth and readjustment. 

5. Endodermic stereids adjacent to canals. 

6. Stereids (pericyclic ?) just within canal in stele. 
7-10. Transverse sections; hypocotyl. 

7. Field seedling, 1.8 mm. diameter. 

8. Irrigated seedling of slow growth. Secondary splitting apart of wood cylinder, 
g. Etiolated seedling 2 mm. diameter. 

10. Etiolated seedling 2 mm. diameter. Hypocotyl through cotyledonary collar. 
Secondary hadrome of cotyledonary median traces is morphologically 
cauline and distinct from primary tissue. /. c. t., Lateral cotyledonary 
trace passing out from the stele. 

loa, lob. Cotyledonary, median traces at another level. 



LLOYD 



PLATE 25 



Tim 
m 




lOb 



136 Guayule. 



Description of Plate 26. 

1. Hypocotyl of an etiolated seedling: the stele, showing rupture of wood col- 

umn and secondary opening of medullary rays. 
2-4. Sectors of the hadrome to show relative amount of conductive and mechanical 
tissue. 

2. Field seedling. 

3. Etiolated seedling. 

4. Irrigated seedling of slow growth. 

5-7. The mechanical tissue (libriform) of hadrome of above. 

5. Field seedling. 

6. Etiolated seedling. 

7. Irrigated seedling. 

b-io. Stereome (leptome) of above. 

8. Field seedling. 

9. Etiolated seedling. 
10. Irrigated seedling. 

I r . Protohadrome of a field seedling ; the size of the lacunse. 

12. Same; base of a peduncle, irrigated 

13. Same; an irrigated plant, 2 cm. from apex. 



LLOYD 



PLATE 26 




o 






o^ 










c 





o 


o 


o 


O 





o 


o 


o 






o 


o 


o 


o 


o 


o 


o 


o 


O 




o 


o 


O 




o 


o 


o 




o 








o 


o 


o 




o 


o 


11 




oooo 







Poo 



o 



oOo 

oooo 

o (? 

0|3 






138 Gitayiile. 



Description of Plate 27. 

1. Hypocotyl, irrigated seedling of very rapid growth, about 2 months old. 

2. Wood of irrigated stem 6.5 mm. in diameter. 

3. Wood of field stem 8 mm. in diameter. (Figs. 2 and 3 are drawn to same scale.) 

4. An irrigated stem. 

5. Growth of 1907, terminal twig of a very large field plant. (Figs. 4 and 5 are 

drawn to same scale.) 

6. Irrigated seedling of very slow growth. 

7. Irrigated seedling of very rapid growth. 

8. Field seedling. (Figs. 6 to 8 are drawn to same scale; the total wood to the 

scale shown, libriform to a larger scale.) 

9. Base of growth of 1908, Cedros, irrigated plant, i year's growth. 

10. 2 years' growth (1906-07), field plant, Cedros. (Figs. 9 to 10 are drawn to same 
scale.) 



LLOYD 



PLATE 27 




140 Guayule. 



Description of Plate 28. 

1. Irrigated plant, stem i year's growth. 

2. Field plant, stem 2 years' growth. Note width of parenchyma rays and depth 

of cortex. (Figs, i and 2 are drawn to same scale.) 

3. Hypocotyl, field seedling less than i year old. 

4. Cortex of hypocotyl after secondary enlargement. Transverse section. 

5. Cortex of root; tangential section. 



LLOYD 



PLATE 28 




142 Guayule. 



Description of Plate 29. 

1. Leptome region of hypocotyl shown in fig. 3. 

2. Leptome region of hypocotyl shown in fig. 4. 

3. Hypocotyl, irrigated seedling of very rapid growth. 

4. Hypocotyl, irrigated seedling of very slow growth. 

5. Pith, Cedros irrigated stem, showing sclerosed pith-cells. 

6. A few of these cells in detail. 



LLOYD 



PLATE 29 




144 Ciiavitle. 



Description of Plate 30. 

1. Hypocolyl, field seedling for station 2, April 1909, less than 8 months old. 

2. First intcrnode, ei)icotyl; the primary stereome bundles. 

3. Periderm opposite bundle i in fig. 2. 

4. Periderm opposite bundle 2 in. fig. 2. 
5-1 1. Trichomes. 



LLOYD 



PLATE 30 




10 



146 Guavulc. 



Description of Plate 31. 

1. Transverse section through upper part of petiole of a well-matured cotyledon, 

in which one sees the ends of the resin-canals, rc^, rcj. Above this level they 
end blindly. 

2. A section of one of the resin-canals (re,) in fig. i, somewhat nearer base of same 

cotyledon. 

3. Transverse section 3 mm. from apex of stem of a field plant, cc. cork. Slow 

growth. 

4. Transverse section through a stem in rapid growth 2 mm. below apex. The 

five medullary canals are established according to a 2/5 phyllotaxy. 

5. A section through a stem apex above that of fig. 4, in which the order of de- 

velopment of the cortical canals is seen to relate to that of the leaves. 

6. Inner part of hadrome bundle of stem, showing cells which become stereids. 

7. The same, in which the stereids are of maximum size and their walls partially 

thickened. (Figs. 6 and 7 are drawn to the same scale.) 

8. Enlargement of stereid elements in leptome previous to thickening of walls. 

Q. The leptome in which the ])rimordial cells which become stereids, .>;/, are seen. 

10. A very young medullary resin-canal in the secretory cells of which are seen 

relatively large globules of rubber. Minute ones appear in adjacent cortical 
cells. 

11. One of these cortical cells on a larger scale, to show the rubber granules more 

exactly. 

12. Secretory cell of medullary resin-canal after periclinal divisions, showing gran- 

ules of rubber. 

13. The schizogenous origin of the medullary canal. 

14. Peridermal divisions in the collonchvma. 



IIOYD 



ClATE 31 




148 Guayule. 



Description of Plate 32. 

6. Sections of resin-canals in which trichome-Hke structures occurred. 

2. Transverse section through a 20-year-old cortex. 

3. Pith-canal with pseudotylose. 

4. Primary cortical canal of the stem with pseudotylose; the spread of periderm 

ipd.) about a stereome bundle is shown. 

5. Pseudotylose in an old cortical canal. 

6. Trichome-like columns formed in pith in seedlings of slow growth. 

7. Primary (endodermal) root canal with pseudotylose. 



LLOYD 



PLATE 32 




150 Guayule. 



Description of Plate t,2)' 

1. Annual rings in duramen of an old stem. 

2. Annual rings in the wood of an 8-year-old stem. 

3-4. Parenchyma rays as seen in a tangential section of a stem of a field plant. 

5-6. Same, irrigated plant. 

7. Sclerosed parenchyma-ray cells of an irrigated plant. 

8. Tracheidal parenchyma-ray cell of a field plant. 

9. Libriform of an irrigated plant; old wood. 
10. Same, field plant; old wood. 



LLOYD 



PLATE 33 




152 Guayule. 



DicscuiJ'TioN OF Plati; ,V1- 

1. Transverse section of a iifdiiiiclf. Tlu' nicihanital tissues are hatched. 

2. Lower epidermis of tlie cotyledon of a field seedling. To accompany figs. 

4 and 7. 

3. Same, sccdlinj^ jjrown in shade (to acconiiiany fi,t,'s. 6 and y). 

4, 7. Cotyledon of a field seedlinj^. 

5, 8. Cotyledon of a seedlinj,' grown in soil with a high saline content. 
(), (). Same, grown with abundant watei and shade. 



LLOYD 



PLATE 34 




154 Guayule. 



Description of Plate 35. 

1-2. First primordial leaf, seedling grown in shade. 

3-4. Same, field. 

5-8. Same, seedling under irrigation (exp. 141, May 1908). 

6-7. Same, in strongly saline soil. 

9. Epidermis of a field plant (Station 2). 

10. Same, irrigated plant. 

11. Stoma of leaf, figs. 6-7. 

12. Transverse section, definitive leaf; field (Station 2, April 1909) 

13. Same, irrigated. August 1908. 

14. Detail of fig. 12. 

15. Detail of fig. 13. 



LLOYD 



PLATE 35 




156 Guayule, 



Description of Plate 36. 

1. Transverse section, lower end of a tap-root after considerable secondary thick- 

ening, to show large size of primary canals. 

2. Same, diarch secondary root. 

3. Detail, to show thin secondary cortex and secondary canals. 

4. Triarch secondary root. 

5. Stem of slow growth. The large size of the cortical canals is notable. 

6. Diagram of the medullary canals at the apex of a slowly growing stem. 

7. Early anastomosis of primary cortical canals, parallel to the plane of the 

cotyledons. 

8. Same, at right angles to the plane of the cotyledons. 



LLOYD 



PLATE 36 




158 Guayule. 



Description of Plate 37. 

1-5. Primary cortical canals. 

1. Endodermal origin in a young epicotyl. This canal has been rotated, but not 

displaced otherwise. 

2. Etiolated seedling-epicotyl. Lateral displacement, but not sufficient to mask 

its relation to the endoderm. 

3. Displacement sufficient to mask origin. 

4-5. First internode. An earlier and later stage in the derivation of the cortical 
canal from the endodermis. The endodermal origin is masked by indefinite 
character of endodermis. 

6-7. Later and earlier conditions of ventral foliar canal, xy indicates the position 
of the hadrome of the mid-vein. 
8. Cortical canal in Parthenium incanum. The endodermal origin is clear. 



LLOYD 



PLATE 37 




160 Guayule. 



Description of Plate 38. 

1. Transverse section, definitive stem, to show primary plan of canals. 

2. Petiole, to show the canals of a large foliage leaf. Dorsal canals of cauline 

origin. Ventral canals, foliar. 
3-5. First foliage leaf. 
6-9. Another (first) foliage leaf. 
10-18. Definitive leaf. 



LLOYD 



PLATE 38 




11 



162 Guayule. 



Description of Plate 39. 

I. Chief (foliage) shoot and axillary bud. Medullary canals in both. 
2-3. Successive planes through chief peduncular shoot, and axillary bud, field 

plant. All canals pass into bud. 
4-7. Successive planes from below upwards of an irrigated chief (peduncular) 
shoot and axillary bud. Two medullary canals which have passed into the 
bud branch to form four canals. 
8. Secondary cortical resin-canals in old cortex. Tangential section. 



LLOYD 



PLATE 39 




CHAPTER VI. 

THE RESIN-CANALS IN THE GUAYULE/ 

THE CANAL-SYSTEMS. 

Because of the comparative interest of the facts involved it is here 
proposed to summarize my observations on the origin, structure, and dis- 
tribution of the resin-canals in Parthenium argentatum. The canals occur 
in this plant in well-defined systems,^ as follows: 

(a) Primary systems: 

1. In the cotyledons, the hypocotyl, and the root, a continuous'' 

system. 

2. Independent of this, the systems in roots of secondary and 

higher order. 

3. In the cortex of the stem and in the dorsal moiety of the 

leaves, forming a continuous system. 

4. An independent* system in the dorsal moiety of the leaf. 

5. An independent* system in the ventral moiety of the leaf. 

6. In the pith of the stem: the medullary system. 
(h) Secondary systems: 

I. Recurrently in the secondary leptome of the root and stem, 
forming continuous concentric systems. There are no 
transverse anastomoses between the several concentric 
systems, such as occur in a laticiferous plant, Manihot 
glaziovii, according to Calvert and Boodle {I.e.). 

PRIMARY CANALS IN THE ROOT AND HYPOCOTYL. 

These have their origin in the endodermis and are included within 
it, as shown for many Compositas by Vuillemin, van Tieghem, Col (/.c.) ,and 
Holm (1908). 

To be noted is a formation of a band of Caspary in the new walls aris- 
ing in the cells destined to become a part of the canal. In the root there 
are two groups, one group of two to four (or occasionally six) canals op- 
posite each primary phloem bundle (plate 36, fig. i) . While this grouping 
is generally true for the Tubulifiorae, the number of canals varies, e.g., in 

' For a summary of the knowledge of the resin or oil canals in the Compositas 
up to 1903, see Col (1903). An excellent historical sketch of the development of 
our knowledge of organs of secretion of oil, resin, etc., is given by Tschirch (1906) 
at p. 1095. 

^ In the usual sense as employed bv, e.g., Vuillemin ('1884b), and by Calvert 
and Boodle (1887). 

3 Vuillemin (18846) properly pointed out the independence of the canals of 
the hypocotyl and epicotyl. He says: "les systemes s^cr^teurs des deux membres 
ou des regions differentes de m^me membre sont toujours distinctes." 

■• As to origin. 

16.5 



166 Guayule. 

Silyhum. Col states that there are six in each group, and it appears from 
his account that the number of those which pass into the hypocotyl is 
scarcely reduced.^ In Parthenium argentatum, however, the number of 
primary canals is usually not more than four; hence it appears that in the 
transition zone the number of canals may be doubled. The four pri- 
mary canals of the hypocotyl pass into the petioles of the cotyledons in 
pairs, there to end blindly (plate 31, figs, i and 2). They do not reach as 
far as the blade. 

The absence of canals in the blade of the cotyledons is to be noted. 
According to Vuillemin, the more numerous canals in the hypocotyl of 
Calendula officinalis pass (in part?) into the cotyledons, on which point 
Col takes issue. Col's figure of the seedling of this species shows groups 
of canals opposite four epicotyledonary bundles, and these he identifies 
with the hypocotyledonary canals, and shows none in association with 
the paired median-trace bundles of the cotyledons. The position in which 
Col's drawing shows the canals suggests that they may be the lower ends 
of the epicotyledonary canals. In many cases, indeed, the true hypoco- 
tyledonary canals may not follow the primary median bundles even into 
the petioles on the cotyledons, while in other cases they may. They may, 
therefore, end blindly in the hypocotyl, by a morphological recedence 
which Col has cleverly traced for the plant as a whole by his extended 
comparative study of numerous Compositas. In Parthenium argentatum 
there are no other canals in the cotyledons (plate 34, figs. 4 to 6). 

PRIMARY CORTICAL CANALS. 

IN SECONDARY ROOTS. 

Primary cortical canals in secondary roots and in those of higher 
orders arise de novo from the endodermis of the new member. This is 
brought about by the morphological independence of the endodermis in 
the roots of diflerent order. Secondary roots are not infrequently triarch 
(plate 36, fig. 4), and have then three groups of canals, two to four in 
each group. In roots, either primary or of a higher order, which grow 
chiefly in length, the canals attain relatively large transverse dimensions, 
and, with a lacunation of the septae between them, there arise columns of 
cells connecting the tangential walls (plate 36, fig. 3). The interpreta- 
tion has been properly applied by Col {I.e., p. 166) to similar appearances 
in Solidago. Col's observations do not, however, negative Vuillemin 's 
previous conclusions, "dans les vieux rhizomes d' Arnica montana, etc.," 
as I point out elsewhere. 

IN THE EPICOTYL AND DEFINITIVE STEM. 

As one ascends the axis the endodermis becomes, as is usually the 
case, a less definite structure. For this reason it becomes increasingly 
difficult to determine with precision the exact origin of the primary corti- 

' Vuillemin (1884a) notes in Silybum a reduction in the number of endodermal 
root-canals by ending blindly, so that a reduced number pass through the hypo- 
cotyl into the cotyledons. The question naturally arises whether the reduction 
in number is not produced by coalescence, as in guayule. 



The Resin-Canals in the Guayule. 167 

cal canals. At the level at which the earliest canals appear, namely, im- 
mediately above the level of the cotyledonary node, the difficulty is not 
as great as higher up. Here the endodermis is evidently involved, and it 
seems conclusive that the whole of the canal structure is derived from it, 
though the cell lineage is not as evident even in a young condition as it is 
at higher levels in Parthenium incanum. This at any rate accords with 
previous observations,' and is without any doubt the case in those parts 
of the stem where the endodermis is regular enough to display its morpho- 
logical relations. I therefore conclude that, were it possible to follow the 
development of the structure, it would be found, even in the higher parts 
of the stem in Partheniufn argentatum, where the endodermis is quite ill- 
defined, to have originated in this. 

The course of development is as follows: A tangential division takes 
place in one, or it may be two or three neighboring endodermal cells. In 
the cell destined to give rise to the canal a radial" division crosses the first 
wall so as to form four cells, realizing the "division cruciaV of van Tieg- 
hem. Periclinal divisions, however, take place, cutting off special secre- 
tory cells, four in number, from a tier of supporting cells, while these suffer 
a still further subdivision. Two pairs arising from the inner two cells of 
the original four are cut off, and are, so to speak, discarded from the canal 
structure, as occurs also in the primary root-canals. Only the outer cells 
of the outer original two become divided, so that fourteen cells in all arise, 
of which four are the original secretory cells, six are the supporting 
cells, and four, or perhaps six, excluded — this in the mariola, Parthenium 
incanum (plate 37, fig. 8). 

The canals of guayule (plate 37, figs. 1-5) bear sufficient resemblance 
to those of the mariola, so that it would be unsafe to deny their entirely 
endodermal origin. Secondary changes, by which the number of secre- 
tory cells as well as that of the supporting cells is multiplied, need not 
be described, as they consist only of repeated radial divisions and some- 
times of tangential ones in the secreting cells. 

These canals suffer more or less displacement (plate 37, fig. 3) accord- 
ing to circumstances, often sufficient to mask their origin. For this reason 
they have been alluded to as cortical b}' Ross (1908) and by Fron and 
Frangois (1901), without raising the question as to their origin. This is. 
perhaps, the reason that, although Col (1903) asserts the endodermal 
origin of the cortical canals in the Tubuliflorae, his drawings sometimes 
fail to show clearly this derivation, as, e.g., in Aster (Bstivalis. 

In Anthemis mixta and Lasthenia glabrata, however, the origin is 
clearly shown, and it seems that the canals are less elaborately organized 
than in Parthenium and suffer less displacement. My own effort has been 
to show conclusively the origin of these canals, with the result that the 
work, in part of Vuillemin, of van Tieghem, and of Col, is supported. 

'Van Tieghem (1884) insists correctly upon the endodermal origin of the 
primary canals, but I am unable to recognize the distinction between canals 
"hordes" and "non-bord^s," though, correlated with the more definite character 
of the endodermis in the roots, the canals are here more regular and soinewhat 
simpler in their structure (but certainly not "non-bordes") than in the stem. 

^ With respect to the stem. 



168 Guayule. 

TOPOGRAPHICAL RELATION OF CORTICAL CANALS. 

The canals of endodermal origin, instead of taking a cortical position, 
may, in various plants, take a position within the pericycle alternating 
with the bundles, or opposite the bundles between the leptome and the 
endodermis. Holm finds such canals in Ambrosia artemisiwfolia, though 
such was supposed to be the case for A. trifida only (Vuillemin, van Tieg- 
hem). In Eupatorium (Holm, 1908), also, canals occur "outside the lep- 
tome." The displacement of the canals and accompanying cells of the 
endodermis to a position nearer the axis appears to have led Vuillemin to 
draw the conclusion that the endodermis of the stem is superposed on 
that of the hypocotyl, an inference which, as Dangeard (1889, p. 122) has 
said, needs confirmation. Vuillemin's figure {I.e., p. 191) is susceptible of 
a different interpretation. 

In the young epicotyledonary axis in Parthenium incanum, the canals 
of the cortex are more usually arranged in pairs, flanking the median leaf- 
traces. This is the permanent arrangement, as, e.g., also in Zinnia (Vuil- 
lemin) and in Olearia haasii (Col, 1903). It comes about, therefore, that 
in the definitive stem of this plant the cortical canals are alternate in posi- 
tion with the bundles. In the guayule they are usually placed on the same 
radius with the bundles, and stand therefore opposite the leptome. Both 
of these arrangements occur in various Compositae.' 

The transition from an alternating position of the canals with respect 
to the bundles in the epicotyl to the radially opposite position presents 
an ontogenetic summary of these two conditions characterizing various 
Compositae in which one or the other arrangement occurs. It may be 
added, however, that the position opposite the bundle in guayule is not 
invariable; exceptionally, canals occur opposite medullary rays. This is 
true of both primary and secondary cortical canals, though Ross states 
the contrar^^ 

ANASTOMOSIS. 

Anastomosis and branching frequently occur between the canals of 
the primary cortical system. The four earliest -formed epicotyledonary 
canals, which arise in pairs associated with the first and second primordial 
leaf-traces, are connected, each with the other canal of each pair, by a trans- 
verse meatus, which lies above the level at which the lateral cotyledonary 
traces pass out from the axis. This transverse meatus is a prominent fea- 
ture of the epicotyl, and is frequently the starting-point of several, usually 
four, canals. Anastomoses in the definitive stem are usually to be found 
at the nodes, and in stems with very short internodes they are frequently 
quite numerous. For this reason, in part, the number of primary cortical 
canals seen in a transverse section varies, as stated by Ross (1908). As 
the stem thickens (aside from secondary changes) the number of canals 
increases, so that from 5 to 20, approximately, may be seen (plate 36, fig. 5). 

' There are very few cortical canals in Parthenium lyratum and in P. hyster- 
ophorus, and they occur on one or both sides of a bundle, but not opposite to it. 
Neither do they stand opposite a medullary ray, strictly speaking, though this 
appears to be the case in P. incanttm. In P. arctium Bartlett they are numerous 
and alternate with the bundles. 



The Resin-Canals in the Guayule. 160 

MEDULLARY CANALS. 

IN THE EPICOTYL. 

All medullary canals are protogenic. Secondary ones do not occur. 
The typical number of canals is not established for ten or more internodes, 
this probably being variable. In field seedlings, or ones of slow growth, 
the distance from the cotyledonary to the tenth node is very short, and 
the particular behavior of the canals is difficult to determine. Etiolated 
seedlings, therefore, throw more light on the matter, though it can not be 
asserted that the behavior in such is always normal, e.g., when the canals 
end blindly above, as they have been observed to do, instead of continuing 
to the apex of the stem. These short canals may, perhaps, be regarded as 
"poches secreteurs" — the pockets in which Col sees reduced canals. The 
following notes, based upon a series of sections made of a seedling about lo 
cm. tall, with i6 nodes, show that the definitive condition is established 
only at length, even the sixteenth node being sometimes reached before 
the full complement of canals occurs. 

No canals below the fifth node. 

At fifth node, one canal passing into bud. 

Fifth intemode, lower part, no canals; upper part, four canals. 

Sixth node, one of these into bud. One branches, making four enter- 
ing lower part of sixth intemode. 

Upper part of sixth intemode, two canals; higher up, three, one send- 
ing a branch to bud of the seventh node. 

Seventh node. At this level two more canals, making five to enter 
the 

Seventh intemode, in which one ends, leaving four in middle part. 

Eighth node, four canals, of which one branches into bud. 

Eighth intemode, two canals in middle part. One branches, making 
three to the 

Ninth node, at which the bud receives a branch. 

Tenth node, four, one branching to bud. All but one end blindly, so 
that the 

Tenth intemode receives only one canal. Two more arise, making 
three for the 

Eleventh node. One passes without branching into the bud, leaving 
two to enter the 

Eleventh intemode. One of these ends, so that one canal reaches the 

Twelfth node, at which one more arises bv branching, and enters the 
bud. 

Twelfth intemode receives one, which ends blindly on reaching the 

Thirteenth node, where a new one arises and passes into the bud. 

Thirteenth intemode has no canals in the lower part. 

Fourteenth node, one canal arises and passes into the bud. 

Fifteenth and sixteenth internodes, two canals in each. 

Despite the irregularity in numbers, and also in position, it is clear 
that the canals in the pith have peculiar relations with the nodes. When 
one arises it does so in connection with the development of an axillary bud, 
and either enters it or sends a branch to it. This is to be inferred also 



170 Guayule. 

from the regular occurrence of live canals, the primary number in the 
growing stem apex. In seedlings with short internodes the canals appear, 
of course, nearer the hypocotyl. In a field seedling 3 cm. tall, with two 
dozen or more nodes, I found one canal at 5 mm. above the hypocotyl. 
The next section cut had one. Similarly in an irrigated seedling with 
short internodes. 

The absence of pith-canals in the epicotyl suggests a primitive al- 
liance with those tubuliflorous forms in which canals are entirely absent 
from the pith. 

IN THE DEFINITIVE STEM. 

At the growing apex within 0.5 mm. one finds a strictly primary ar- 
rangement of these canals. There are five, one opposite each orthosti- 
chy.^ In a slowly growing stem, however, in which the nodes are crowded 
upon each other, through frequent branching and anastomosis, the num- 
ber seen will vary usually between three and six. The union and separa- 
tion of the canals is associated with the formation of large lacunae giving 
ofi[ large passages of irregular shape, but on the whole running longitudi- 
nally. In a single section, therefore, one may count as many as a dozen 
canals, and nearby as few as three or four. In rapidly growing shoots the 
anastomoses and branches are not so apparent, though they occur here 
also. From the canal nearest to it each bud receives normally a single 
branch, which, itself branching after entering the bud, increases till the 
complement is reached. Pith-canals do not enter the leaf. 

TOPOGRAPHIC RELATIONS OF MEDULLARY CANALS. 

Although the primary number of pith-canals is more or less masked 
by branching and anastomosis, as already mentioned, a study of the on- 
togeny of the stem can not fail to show that five is the primary number 
(plate 38, fig. i) , and further, that they arise in the same order as the leaves 
and, therefore, buds. These relations are seen best in growing tips of stems 
of not too slow growth, or in seedlings, just above the levels at which the 
pith-canals first come in. It is also evident from the positions taken by 
the solitary canals which appear in the epicotyl before the full complement 
is established. 

The very frequent anastomosis and divarication, coupled with the 
transverse expansion of the canals, give rise to a great many columnar, 
trichome-like structures, already alluded to. They lie approximately 
in radial planes, and can be explained only as imperforate longitudinal 
septae (plate 32, figs. 1,6). 

In older stems the breaking down of the pith results in the opening of 
the resin-canals, except when plugged by pseudotylbses. There results a 
downward filtration of resin which finds its way into the central zones of 
the old wood. This often becomes richly impregnated with resin, though 
primarily it contains none at all. In this way the resin-content of old 
wood, shown by chemical methods, is to be accounted for (Lloyd, 1909). 

' In the pith of Cynara carduncula (Col, 1903) 5 to 10 canals occur; in 
Parthenium hysterophorus I count about 20; in P. lyratum about 12. 



The Resin-Canals in the Guayule. 171 

THE CANALS IN THE LEAF. 

Since the canals in the leaves are related only to the primary cortical 
system, this relation will now be taken up. 

EARLY FOLIAGE LEAVES. 

The above-mentioned pair of primary cortical canals which enter the 
petioles of the earlier leaves end blindly at different levels in the petioles 
and in the leaf-blade^ (plate 38, figs. 3 to 9). The marginal leaf-traces 
enter the petiole unaccompanied by canals, but arise de novo in the petiole 
dorsal to the lateral traces. These they follow into the leaf-blade, and 
branch, constituting a latero-dorsal system. The dorsal system may be 
entirely absent from the blade (plate 38, fig. 5). There is also a ventral 
system composed of three canals, one opposite each of three prominent 
bundles, namely, the median and two lateral. These arise independently 
and de novo, that opposite the median trace in the petiole, and those oppo- 
site the lateral ones, in the blade. They originate analogously with the 
pith-canals, independently of the endodermis^ (plate 37, figs. 6, 7). 

THE ILATER^LEAVES. 

The later leaves, in which their definitive character is assumed, re- 
ceive usually three to five (occasionally six or seven) cortical canals from 
the stem, one with the median and two with each of the stronger lateral 
traces (plate 38, figs. 2, 10 to 18). These canals, which enter the blade, 
follow the traces which constitute its prominent veins. The lateral 
canals may branch, usually not more than once. Thus the dorsal system 
of canals has, at most, usually not more than five ducts. The median canal 
follows the midrib to the apex of the leaf. The lateral ends some distance 
from the apex. The ventral system arises de novo in the petiole as three 
to five independent ducts (plate 38, figs. 10 to 13) , the median arising first. 
The lateral canals follow the main limbs of the lateral traces and give off 
branches to veins of a higher order, until, in a transverse section, there 
may be five or more on each half of the blade. It is thus seen that the 
ventral system is peculiar to the leaf and is more extensive than the dorsal 
system. The canals anastomose in the upper part of the blade and follow 
the veins. 

PRIMARY CANALS IN BRANCHES. 

The primary system of cortical canals in a branch is derived from two 
canals on either side of the appropriate leaf-trace. At the level at which the 
bud appears, the adjacent canals in the chief stem enlarge radially and send 

' The behavior described is not invariable. One case was found in which 
only one branch of the canal anastomosis entered the first leaf, while the second 
leaf was normal, having two canals. The third foliage leaf in this plant also had 
but one canal. One instance of a leaf at about the twentieth node had two canals. 
This condition offers an analogy to that in the cotyledons, which may be held, 
though only tentatively, as speaking for the more primitive character of the 
double arrangement. 

' It is worth noting here that there is a single ventral canal opposite the mid- 
vein in the cotyledon of the common sunflower, Helianthus annutis. 



172 Guayule. 

off a number of branches which distribute themselves in the cortex of the 
bud. As already said, generally a single branch from the pith-canal oppo- 
site the bud enters and branches to produce the complement of canals 
(plate 39, figs, i and 7). 

SECONDARY CANALS IN ROOT, HYPOCOTYL, AND STEM. 

These arise, as described by Ross, from special leptome parenchyma ' 
derived directly from the cambium, and quite in the same way in all parts 
of the plant. They are at first flattened radially, opening out later to be- 
come rounded or even circular in transverse section, and finally becoming 
again flattened and secondarily distended, in company with the growing 
(secondary) cortex (plate 22, fig. 13). These canals constitute concentric 
branching and anastomosing systems, each succeeding zone being a sys- 
tem separate from all the others. Their appearance in tangential sections 
(plate 39, fig. 8j recalls the figure published by Tschirsch (1906, p. 11 93) 
of the canals in wound-tissue in Larix. 

CANALS IN THE PEDUNCLE. 

It has already been pointed out that the inflorescence is terminal; 
the peduncle is therefore the morphological chief shoot. I have shown 
that when an axillary bud develops it usually receives one canal from the 
pith (plate 39, fig. i). The last bud formed on the chief shoot which ends 
in a peduncle, however, receives all of the canals from the pith, these being 
diverted en masse. The peduncle, therefore, contains no medullary canals 
(plate 39, figs. 2, 3). Primary cortical canals alone occur, there being but 
very little secondary thickening. 

Exceedingly interesting relations in this regard are displayed by 
rapidly grown plants (plate 39, figs. 4 to 7), In another chapter two types 
of guayule have been described, in one of which the sharp delimitation 
between peduncle and foliage stem is not present. When guayule is irri- 
gated there frequently results, associated with rapid growth, a tendency 
of the relatively chief shoot to run out into inflorescence,^ when otherwise 
there would be a sharp transition from stem to peduncle, and the upper 
axillary bud would develop strongly. When the morphological transition 
is gradual, there is also a correlated anatomical transition, which the long 
intemodes make it possible to analyze. In a specimen examined, as in 
the normal condition, the peduncle has no pith-canals, but the first inter- 
node below this has, instead of five, only two, which pass into the upper- 
most axillary bud. 

The sector of the stem under the peduncle contains much more stere- 
ome, and the two canals are confined to the sector beneath the axillary 
bud, while from the basal part of the internode they are absent! Their 
orientation above is such as to bring them opposite the first and second 
leaves of the axillary bud; they are, therefore, the canals which give 
branches to the first two axillary buds of the branch. 

The axillary bud of the second node below the peduncle receives from 
the stem one canal onlv of four which are to be found in the internode be- 



' Secondary leptome-canals have been described in Centrophyllum lanatutn 
(Col, I.e.). 

' Simulating the normal shoot in P. incatium (mariola). 



LLOYD 



i^'.' 


aar 


' t:. ' 


ir 




^ 




1 . Rubber in canal cells, nearby cortex and inner 

ray cells. Root 1 .2 mm. diam. 

2. Older root. More rubber in rays. 

3. Root 2 mm. diam. 

4. Parenchyma ray from fig. 2. 

5. Upper part of hypocotyl, same age as fig. I . 



6. Longitudinal section through old wood. 

7. Longitudinal section through mature leptome 

parenchyma, with a few parenchyma ray cells. 

8. Leptome; elongated elements. 

9. Companion cells and sieve tubes. No rubber 

in younger leptome on the left. 



PLATE 41 









•6 



2. Cortex, stem of field plant with maximum rubber content. 

3. Cortex of a 20-year-old stem. 

4. Root; rapidly grown seedling, two months old. Rubber in granules. 

5. Rubber in process of accumulation in an irrigated plant. 

6. Primary resin canal, root I mm. diam. 



The Resin-Canals in the Guayule. 173 

low the second node. The other three end blindly before they reach the 
node, so that the following internode has none, as above said. It is evi- 
dent that we find here a sort of morphological indecision, as if the stem 
were trying to retain its stem character, and still being gradually over- 
come by the tendency toward changing into a peduncle. The same prepa- 
rations show also the formation of chlorenchyma strips in the cortex of 
the peduncle sector, nearly down to the base of the first internode below 
the peduncle. 

The axillary bud of the third node below the peduncle receives a 
single branch from one of five canals, the normal number, present in the 
internode below. Here, therefore, the complete stem structure is first 
met in our descent from the peduncle. It would be interesting to specu- 
late on the internal causes which result in diverting the canals, en masse, 
from the chief shoot into an axillary bud. 

THE CANALS IN RETONOS. 

New shoots which take their origin from roots have this peculiarity 
in common with the epicotyl, that they do not possess medullary canals 
till several internodes have developed. They are further peculiar in lack- 
ing primary cortical canals near their bases. A retono 23 mm. long was 
examined and measured. A section near the root at the level of emerg- 
ence showed neither pith nor cortical canals. At 5 mm. above it five cortical 
canals were found. At 10 mm. there were three medullary canals, and at 
1 5 mm. five of these, so that at the level of 1 5 mm. the definitive structure 
had been attained. 

In another specimen 25 mm. long, collected September 8, 1908, only 
one medullary was found at the level of 18 mm., and four at 21 mm. In 
still another, one canal was found at 15 mm. 

An examination of a full series of transverse sections through suc- 
ceeding nodes and internodes discovers an important relation of the pith- 
canals to branches, in general harmony with the facts cited immediately 
above. The material thus studied was a retono several centimeters long 
which developed in 1908. The first leaf and its axillary bud were devel- 
oped at the height of 20 mm. The internode between the mother-root and 
this node had no pith-canal. At the first node a single canal appeared just 
below the level of the bud, and entered this. The succeeding two inter- 
nodes (second and third) were also devoid of canals, though at each of the 
corresponding nodes a single canal originated in the pith and passed into 
the axillary bud. At the third node, however, the canal branched, one 
limb passing up into the fourth internode, in the upper part of which two 
other canals appeared. One of these three sent a branch to the bud of the 
fourth node, and one ended blindly, leaving two passing into the fifth 
internode. At the fifth node one of these sent two branches into the bud, 
two canals passing into the sixth internode. At the sixth node both of 
these branched, one branch going into the bud and three upward into the 
seventh internode. At the seventh node all three branched, one of these 
going into the bud, leaving the full complement of five canals for the suc- 
ceeding internode , the eighth . The youngest canal always stands opposite 
the youngest bud. 



174 Guayule. 



THE CONTENTS OF THE CANALS ; THEIR ORIGIN. 

The very small size of the primary canals in the root and hypocotyl 
makes it very difficult to determine the nature of their contents. The 
canals elsewhere are known to contain resin which, upon wounding, exudes 
as tears, which fall to the ground and harden slowly as pale yellow, limpid 
masses. The origin of this secretion is of special interest here. There is 
no doubt that the resin is confined to the canals, and there is no evidence 
that the resin occurs in the protoplasm of the wall-cells of the canal, 
which have been spoken of as secretory. Treatment with alcohol or with 
acetone leaves the cell-contents quite unchanged to all appearance, though 
subsequent staining with alkanet discloses, when this is originally the case, 
a substance which may be dissolved out by means of xylol or other appro- 
priate solvent, namely, rubber. My own observations, therefore, give 
support to the general view, advanced by Tschirch, that the resin is to 
be accounted for by chemical activity in the outer part of the cell-walls 
facing the meatus. It is not a direct result of protoplasmic activity, but of 
enzymatic activity in the cell-wall itself.^ It is worthy of remark that the 
wall (secretory) cells of the resin-canals have the two-fold function of secret- 
ing rubber (in common with the ground-tissue) within the protoplasm and 
resin without. 

I have, however, attained no success in demonstrating a mucilaginous 
or gummy lining to the meatus, such as is described by Tschirch (1906, 
p. 1 1 19) in many plants, to which he ascribes the origin of resin formation. 
But Tschirch himself confesses to a similar difficulty in studying, among 
others, the Compositae. 

The distribution of starch in the cortex and its apparent connection 
with the secretion of resin have been elsewhere noted. The presence of 
tannin in the conjunctiva of the young stem, especially associated with the 
chloroplasts, is to be noted, and recalls Tschirch's hypothesis of the origin 
of resin from tannin. The number of Compositse which contain tannin 
is small, relatively to the size of the group, judging from the list given 
by Dekker (1906). 

THE ROLE OF RESIN. 

It has often been pointed out^ that resins and ethereal oils stand in 
relation to climatic conditions, especially those of the desert. The frequent 
occurrence of resin in desert plants is a matter of general observation, but 
its function is still a matter of speculation. Tschirch rightly lays stress 
upon the occurrence of secretion-containing structures near the apex of 
the young parts as of significance, and this has been pointed out for 
the guayule. The evidence regarding the relation of resin to rubber 
leads us nowhere, and no evidence is yet forthcoming as to the real role 
of resin. 

* Tschirch, A. Die Chemie und Biologic der pflanzlichen Sekrete. Leipzig, 1908. 
^ e.g., Tschirch, 1908, pp. 8-q. 



The Resin-Canals in the Guayule. 175 

RESIN-CONTENT OF GUAYULE BY ANALYSIS. 

The percentage of resin in the branches and twigs of field plants, 
according to figures obtained by Whittelsey (in manuscript) , is between 
about lo per cent for the smaller and about 17 per cent for the larger 
branches. The amount probably varies according to the structure, and 
this with the rate of growth of the parts. For irrigated plants the follow- 
ing figures were obtained. The material was the same as that referred to 
in table 53. 

Table 51. 

Percentage 
Parts. of resin. 

I. Stump 2.46 

Ila. Wood of 1907 growth 1.36 

116. Cortex of this 4 . 06 

in. Growths of 1908 intact 7 . s6 

IV. New growth of 1909 with leaves 2 . 70 

V. Roots I o . 80 

Aside from possible errors, it seems that, bulk for bulk, the irrigated 
plant contains less resin than the field plant. This is due in part to the 
larger relative volume of the wood cylinder. The reduction of the amount 
in older growths is due also in part to the radial compression of the resin- 
canals in irrigated plants, whereby their capacity is much reduced. The 
force of this explanation of the figures appears when we compare the per- 
centage of resin in III above. When we introduce the rate of growth as 
a factor we must conclude that the total secretive activity is not reduced 
under irrigation, nor is the secretive activity of the resin-secreting cells 
lowered. The result, however, is had that in a given volume of cortex 
there is less resin in irrigated plants. In the pith, however, this does not 
hold, since the relative volume of the resin-canals under irrigation is as 
great or greater than in field plants. The reduced amount of resin of the 
cortex , volume for volume , appears , therefore , to be a secondary matter only , 
and bears, so far as we can see, no explanation in terms of adaptation. 



CHAPTER VII. 
THE ORIGIN AND OCCURRENCE OF RUBBER.^ 

Well-nigh nothing is known about the cytology of rubber-secreting 
cells. The great initial difficulties in the investigation have arisen from 
the fact that in most rubber-producing plants this material occurs in 
latex. In the guayule, as in a few other known plants, the rubber is laid 
down within certain cells, in a manner analogous to the formation of starch. 
Although the study of the early cytological activities which lead to the 
accumulation of rubber still presents great difficulties, since some of the 
agents used dissolve out the rubber, nevertheless it has been possible to 
determine the relation of growth and of some of the more important ex- 
ternal conditions to rubber secretion. These results are important eco- 
nomically, since we are able to determine the time at which the maximum, 
or near the maximum, amount of rubber occurs, and during what period 
rubber is absent from the new tissues, and thus establish rules of pro- 
cedure in the harvesting of the shrub. 

METHODS. 

The solubility of rubber in xylol and the like prevents the use of 
paraffin. The preparations must therefore be studied in such a fashion 
that the rubber is intact. When present in large quantities it is easily 
recognized, after one has become acquainted with its appearance.^ When 
in small quantities, however, it may easily be mistaken for droplets of oil 
or resin, or for protoplasmic or other granulations, and inasmuch as oils 
and resins as well as rubber are stained by alkanet, these substances, if 
present, must be removed by suitable solvents which will leave the rubber 
unaffected. For this purpose I have treated sections with high-grade and 
absolute alcohols, acetone, and potassium hydrate, applying alkanet both 
before and after. There remains the possibility that the substances which 
remain and which react to alkanet are not always rubber in its final form, 
but there can be little doubt that the materials which are referred to below 
are either rubber or are substances in the course of change into rubber. 
The evidence seems to indicate, however, that it is rubber which we are 
dealing with. 

In seeking to determine with accuracy the facts of the distribution 
of rubber in the tissues, the accident of displacement of rubber in the act 
of sectioning must be properly guarded against. When rubber is present, 

* The substance of this chapter was presented in a paper entitled "The 
responses of the guayule, Parthenium argentatum Gray, to irrigation," before the 
Botanical Society of America, Boston, December 1909. 

' When in readily appreciable quantities, resin and rubber in the guayule may 
readily be distinguished by alkanet. Resin takes on a brilliant scarlet, while rubber 
has a purplish tinge, and is, to the naked eye, blood-red. 
176 



LLOYO 




^^ / 


PLATE 42 




»■ 













1 . Apex of terminal twig of 1 908, field 

plant, July 22. 

2. Near base of same. 

3. Pseudo-tylose with rubber in the cells. 

4. Leptome, field plant. 




5. Pith of a field stem, 10 mm. diam. 

6. An old leaf trace. 

7. Outer cortex of a field stem. 

8. Outer edge of cortex and inner zone of 

cork derived from coUenchyma. 



The Origin and Occurrence of Rubber. 177 

the contents of the cells ruptured during the sweep of the knife agglomer- 
ate and stick in irregular masses to the section. It is also to be suspected 
that particles of rubber displaced from one cell may remain attached to 
other cells in such a manner as to simulate an original position in them. 
This danger is greater where the particles are small, since with smaller 
size the chance against agglomeration is greater. With some experience, 
however, this general difficulty is reduced, so that, with proper observa- 
tion, mistakes are easily avoided. The guiding principle of observation is 
simply to confine study to uninjured cells. 

GENERAL DISTRIBUTION OF RUBBER IN THE PLANT. 

It has been known for some years ' that the rubber in guayule occurs 
in the parenchyma " cells " of the stem and root ; that is, in the pith, paren- 
chyma rays, and cortex (the conjuctiva, in a word). These facts, though 
known to a few, were first clearly stated by Ross in 1 908, according to whom 
rubber occurs in almost all the cells of the ground-tissue in root and stem ; 
that is, in those of the pith, parenchyma-rays, primary cortex, and also in 
the w^ood-parenchyma. The leaves, he adds, contain little or none. While 
I have been able to confirm these conclusions in general, several additional 
details have come to light. 

Rubber occurs invariably in all the cells of the resin-canals (plate 41 , 
figs. 4-6) . While I am unable to state positively that it occurs here earlier 
than elsewhere, it certainly is secreted most rapidly. There is, however, 
evidence that the former statement is true. 

In the primary hadrome parenchyma rubber does not occur early. 
In the preparations from which the photographs on plate 42 (figs, i and 2) 
w^ere taken, there was no trace of rubber. There can be no doubt, how- 
ever, that rubber is secreted by some or all of these cells later on, as they 
are replete after some secondary thickening has occurred in the leaf -trace, 
which, of course, suffers no secondary change^ (plate 42, fig. 6). The cells 
of uniseriate parenchyma rays, in consonance with other parenchyma-ray 
cells, contain rubber. 

In the primary leptome, under the conditions noted for the primary 
hadrome in the preceding paragraph, rubber occurs at least in the fiber- 
cells and in the parenchyma (plate 42, fig. 6). This takes place after the 
abscission of the corresponding leaf. In secondary leptome I have seen 
small amounts in mature fiber-cells, just before sclerosis sets in. Occur- 
rence of rubber in these elements appears, therefore, to be a function of 
age. It is secreted normally in leptome-parenchyma, and at the same 
time is in adjacent parenchyma-ray cells (plate 42, fig. 4). 

In the secondary leptome rubber may also be seen in all the elements 
of the sieve-tissue (plate 40, figs. 8, 9). It is true that the very narrow 



' Fron and Frangois, 1901. 

' The parenchyma of the secondary hadrome is rather scanty and of small 
elements. They do not secrete rubber as early as the medullary-ray cells in the 
same zone, but ultimately do so. The rubber may best be seen in longitudinal 
sections, treated with boiling 10 per cent caustic potash and stained with alkanet. 
The rubijer then appears as small series of globules. The wood may also be macerated 
by means of Schultz's medium, and later stained. 
12 



178 Guayule. 

elements contain very little, but this amount may be clearly demonstrated 
in longitudinal sections treated as above described. 

In the cork-cells rubber occurs in a secondary condition as small 
droplets, derived by the breaking up (possibly an emulsification) of the 
compact masses in the outer cortex. These droplets are larger in cork-cells 
on either side of collench3'matic zones which are remnants of the periclinal 
walls of coUenchyma (plate 42, fig. 8). 

Rubber is secreted in the parenchyma of the pseudotyloses (plate 42, 
fig. 3), quite as in the adjacent cells. 

In the leaf the amount of rubber, though always small, is in propor- 
tion to its age. In the oldest leaf I have observed, rubber occurs in drop- 
lets in the outermost palisade-cells of both surfaces, and less conspicuously 
in the subjacent, but usually in no other chlorenchyma cells (plate 43, fig. 
5). It may occur only in the ventral palisade in younger leaves. In very 
minute droplets it is to be found also in the collenchyma and endodermis 
of the midvein and in nearly all of the non-chlorophyllous cells in the 
region about it, and in the leptome, both in the companion and sieve cells. 
Curiously enough, it is not to be found in the secreting-cells of the resin- 
canals, though, on the other hand, it is in small but conspicuous quantities 
in the subjacent cells. A minute amount occurs in the epidermis, espe- 
cially near the midvein, and in the non-chlorophyllous cells near the smaller 
veins. The maximum quantity, negligible from the economic point of 
view, occurs in the oldest leaves which have passed through a drought 
period. The material which gave these results was collected in the spring 
of 1909 before the summer rain of that year. 

In material collected from irrigated plants at Cedros in April 1909 
rubber may be detected in exceedingly minute quantities in the basal part 
of the leaf only. A single minute droplet — not more than one-fourth the 
diameter of those seen in the field plant — may be seen in each outermost 
palisade-cell of the upper (ventral) surface. They are a trifle larger near 
the midvein. In the non-chlorophyllous tissue near this the rubber may 
also be detected in still more minute quantities. 

Since within the periphery of the wood cylinder only the conjunctiva 
and a small amount of wood-parenchyma contain rubber, and since in older 
wood the medullary rays (in part) and the pith and its canals are dead 
and disintegrated, the wood cylinder contains less rubber than the cortical 
tissues, but it is also less resinous in its primary condition. Recent work 
by Whittelsey (1909), however, indicates that in stems of an advanced 
age, at any rate, the amount of true rubber is practically reduced to nil, 
though in the young twigs the proportion of rubber within the periphery 
of the wood cylinder is large. We must conclude, therefore, that the 
rubber in the older wood undergoes chemical change, and is broken down 
into related materials. There is no doubt that some such change takes 
place also in the secondary cortical tissues cut out by the inner periderm, 
and this, as I have shown, is a considerable part of the volume of the 
"bark" in older stems. 



The Origin and Occurrence of Rubber. 179 

APPEARANCE OF RUBBER IN RICHLY LOADED TISSUES. 

A section taken through any young stem of a field plant after some 
period of drought will give a typical appearance (plate 42, fig. 7). All the 
cells of the conjunctiva appear to be filled with a gray substance. A good 
deal of it will have been swept out of ruptured cells by the knife-edge and 
agglomerated, the resulting masses having irregularly rounded outlines, 
with strands stretching here and there, still attached to the tissue. These 
masses, seen obscuring the pith to some extent in plate 43, fig. 2, and the 
dense cell-contents stain deeply with alkanet, the stain being more bril- 
liant if the sections have been previously boiled in a 10 per cent solution 
of caustic potash. If the sections have not been acted on by alcohol or 
potash drops of yellow resin will be seen in the resin-canals, but nowhere 
else, except accidentally. 

Closer examination of the rubber within the cells shows that the mass 
is not homogeneous, and does not entirely fill the cavity. It may form a 
heavy layer about the wall, leaving a more or less irregular space within, 
or, if apparently filling the entire cell, it will contain numerous spherical 
spaces (plate 41 , figs. 1,2). Sections which have lain in glycerin may show 
the masses tobe contracted, owing to a plasmoly tic action upon them, from 
which it is to be inferred that they have a considerable water-content, 
held within the vacuoles, in part at least (plate 41 , fig. 3) . The rubber may 
also accumulate as a round drop within the vacuole of the cell (plate 42, 
fig. 7), its size depending upon the age of the cell. Plasmolysis shows 
further that all the parenchyma cells are not equally densely filled, though 
of the same age. This is often conspicuously the case when the cells of 
the cortex and those of the adjacent parenchyma rays are compared. In 
the cortical cells the rubber forms a dense rounded drop (plate 42, fig. 7), 
while the cytoplasm may be seen between it and the cell wall. In paren- 
chyma-ray cells the rubber mass is frequently irregular, full of irregular 
vacuoles, and the cytoplasm appears usually to have shrunk away with 
it. In the parenchyma-ray cells in some preparations it is quite as regular 
as in the adjacent cortical cells, but appears to be more dense, owing to a 
very much larger number of minute spaces. This difference, in one form or 
the other, is quite constant, and seems to indicate that the rubber-content 
of the cortical cells is higher than that of the adjacent parenchyma-ray 
cells. 

Cortex which has been cut out en masse by inner periderm also con- 
tains rubber. In the cells of this tissue it has a still different appearance, 
being segregated into droplets of various sizes, in a fashion to suggest the 
analogous appearance. of dead protoplasm. In newly formed cork-cells 
proper, just outside of the periderm, a different behavior is seen. 

BEHAVIOR OF PERIDERMAL DIVISIONS TOWARD RUBBER. 

Since the secondary cortical cells in field plants contain a large 
amount of rubber in the condition described, the fact that the cork-cells 
immediately outside of the actively dividing suberogenous cells may contain 
no rubber at all, or only occasionally a small amount, calls for explanation. 
The suberized walls of the cork take up alkanet readily, so that, after 



180 Guayule. 

treatment with that reagent, the contrast between the rubber-containing 
cells of the cortex and the empty nearby cork-cells is very clear and striking. 
Inasmuch as the peridermal divisions, though several times repeatedinthe 
same mother-cell, finally involve a considerable depth of tissue, and as 
the rubber can not travel from cell to cell as such, we must conclude either 
that the rubber is translocated, which is unlikely, or that it disintegrates. 
In support of the latter conclusion we note the following ocular evidence: 

1. When the first cork-cambium division takes place the partition 
passes through the rubber-content, whereby the two daughter-cells each 
receive a share (plate 31, fig. 14). From the outer cell, which becomes 
suberized, the rubber disappears. 

2. This disappearance is gradual. The rubber may first break up 
into droplets, which become fewer in number till, in the second series 
of cork-cells, scarcely any evidence of its former presence remains, or it 
may become shrunken in appearance. During this time the rubber, if it 
still is such, reacts less characteristically to alkanet, and takes on a dirty 
bluish tint. In one young root, however, I observed droplets of rubber 
giving the characteristic stain, out several cells distant in the cork. The 
explanation may be that after the death of the protoplasm the oxidizing 
enzymes present hasten the disintegration. This may be less rapid in the 
root, though it is difficult to say why. The mere contact with the air 
would seem an insufficient explanation, since disintegration of the rubber 
in cortex cut out bodily by inner periderm is very slow. 

THE DEVELOPMENT OF RUBBER IN THE CELL. 

All that we are able to do microscopically in regard to the method of 
origin of rubber in the cell is to detect its first appearance and the subse- 
quent accumulation, and we are therefore precisely in the position of 
the poet who said of a matter usually regarded as far removed from the 
realm of science, 

"Sie kommt, und sie ist da." 

We are unable to say at this point whether the origin is associated with 
special organs as plastids or not, though my observations up to the present 
indicate that there are no such organs. 

The relation of nuclear activity in general to secretion is well known. 
The rubber in the paUsade-cells of the leaf appears first in all cases in 
contact with the nuclear membrane, and for this reason does not take the 
form of spherical but of concavo-convex droplets, seen in plate 43, fig. 5. 
Elsewhere the earliest appearance is as very minute, well-nigh invisible 
droplets (plate 41, fig. 4), scattered in the protoplasm. They grow in 
size and increase in numbers until the protoplasm is loaded sufificiently 
to render it exceedingly frothy in appearance (plate 41, fig. 5). These 
droplets may travel toward the interior of the cell and be extruded into 
the vacuole, where they run together to form a larger droplet or a more 
or less irregular mass. This is not homogeneous, as might be supposed, 
but is vacuolated, sometimes so much so that it is quite alveolar in struc- 
ture (plate 41, fig. i), sometimes less so, the vacuoles being widely scat- 



The Origin and Occurrence of Rubber. 181 

tered. That these vacuoles contain various substances in solution in the 
inclosed water can not be doubted, and it seems likely that among these 
are enzymes'^ which may act upon the rubber after extraction by the 
mechanical processes in vogue. It also seems likely that the protoplasm 
of the cells becomes intermingled with the rubber during extraction, ren- 
dering it more or less albuminous and liable to give off the products of the 
decay. 

CENTERS OF SECRETION. 
THE ROOT. 

With certain exceptions, the secretion of rubber both in the stem 
and the root, including the hypocotyl, appears to proceed from definite 
centers. This is exemplified with especial clearness in the root, where, in 
the cortex, the secreting-cells of the resin-canals^ are the first to show the 
presence of granules of rubber (plate 41, fig. 6). It is argued that secre- 
tion actually begins earlier in these cells because the surrounding cortical 
cells, primary on the outside, secondary on the inside, contain, at an early 
stage of secretion, less and less rubber, as one proceeds farther from the 
canals. The figures of plate 40 illustrate this advance in secretion, the 
beginning of which is seen in a young stage in the development of the 
root (plate 23, figs. 3, 7; plate 40, fig. i). If the rate of growth has not 
been too rapid, so that a part of the primary cortex has had the necessary 
time to secrete rubber before being cast off, the activity of secretion is 
seen to be taken up successively by the cells further removed, until the 
whole tissue becomes loaded (plate 40, figs. 2,3). The greater amount of 
rubber, however, is evidently held by the cells nearer the resin-canals. 
In the hypocotyl the same physiological relations hold. 

The secretive activity of the secondary cortex is taken up, aside from 
those cells in the neighborhood of the canals, by successive layers of cells, 
beginning on the outside. With the appearance of the secondary resin- 
canals, however, a superior activity in rubber secretion in their secreting- 
cells is to be early noted. 

On the other hand, simultaneously with the appearance of rubber in 
the primary canal-cells, it appears also in the innermost cells of the paren- 
chyma rays, the function of secretion being taken up successively by the 
next outer cells, and so on. This is apparent in the figures (plate 40, figs, i 
to 4). If a period of rapid growth follows one of stasis, the newly formed 
parenchyma-ray tissues will show an entire absence of rubber (plate 40, 
fig- 3)- When secretion again begins, it starts simultaneously in the 
outermost and innermost cells of the parenchyma ray. 

THE HYPOCOTYL. 
In the hypocotyl a similar condition prevails, though here, as in the 
definitive stem, there is a pith. That is, the innermost parenchyma-ray 
cells assume secretive ability earlier than the pith-cells, which is not true 
for the definitive stem (plate 40, fig. 5). 

' The presence of oxidases in extracted rubber, both in latex rubbers (Spence, 
1909) and in guayule rubber, is known. 

- In view of the emphasis which has been placed by many writers on the endo- 
dermis as seat of high physiological activity, the beginning of the secretion of rub- 
ber in the resin-canal cells, which are constituents of the endodermis, is of very great 
interest. 



182 Guayule. 

THE STEM. 

In the stem, the first evidences of rubber are to be observed in the 
secreting-cells of the cortical and medullary canals simultaneously. The 
dark appearance of these cells in figure i, plate 42, is due, in part, to their 
larger rubber-content, but in part to the denser protoplasm. The condi- 
tion to be seen in these cells is represented by the camera drawing in plate 
31, figs. 10 and 12. The section was taken toward the apex of a newly 
grown twig of a field plant, collected on July 22, 1908, and was then about 
six weeks old. In all the cells of the conjunctiva very minute granules of 
rubber could be seen, but not more in the cells near the canals than else- 
where. In the stem, therefore, secretion appears to begin first simultane- 
ously in the canal-cells of the pith and cortex, and then in the conjunctiva. 
It is, however, quite readily determined that the physiological activity of 
the pith is greater than that of the cortex. In fig. 2, plate 42, is shown 
a section taken from the twig just mentioned, but near the base of the 
new growth. One or two peridermal divisions have ensued, while other 
secondary changes may be noted. The rubber-content of the pith-cells is 
obviously greater than that of the cortex in this section. Further, I have 
noted in irrigated plants that the amount of rubber is greater in the outer 
than in the inner cortical cells (plate 43, fig. i). It seems, therefore, that 
the deportment of both root and stem is essentially the same and that the 
hypocotyl, though possessing a pith, behaves as the root. 

During secondary thickening, as in the root, the secondary cortical 
canals exhibit early activity in rubber secretion, while this is taken up by 
the oldest parenchyma-ray cells first, simultaneously, therefore, at the 
inner and outer edges. 

THE LEAF. 

In the leaf the earliest appearance of rubber is in the outer palisade 
in the ventral moiety. I found rubber in these cells only in old leaves of 
irrigated plants. The analogy with the condition described for the stem,, 
in which superior activity is shown by the pith, is clear. But the failure 
of the leaf-canal cells to show greater activity than the neighboring con- 
junctiva detracts from the force of the comparison. The leaf observed 
by me to be most richly supplied with rubber contained a single drop- 
let, with a diameter about half the transverse diameter of the cells, in 
each palisade-cell toward the median vein. The amount of the rubber 
became less and less toward the margin. This was true also of the outer 
palisade of the dorsal (lower) surface, and in a less degree of the inner 
palisade. 

Minute granules occurred also in all the non-chlorophyllous cells, 
mechanical and conjunctive, forming the midrib, excepting the vascular 
and sieve elements. It would seem, therefore, that, roughly speaking, the 
midvein is the center of rubber secretion, which proceeds through the 
lamina toward the margins; further, that activity is shown first by the 
outer palisade-cells, then by the inner, and first by the ventral and later 
by the dorsal. In this regard, as already said, the analogy to the stem is 
clear. 



The Origin and Occurrence of Rubber. 1 83 

RATE OF RUBBER SECRETION RELATIVE TO GROWTH. 

The material which I have studied in order to determine the relation 
of growth to the rate of rubber secretion was collected during and follow- 
ing the growing-season of 1908, which began about June i. Growth is 
rapid for the first part of the season, during which several centimeters of 
stem-length are attained and one to three flower-stalks are developed. A 
period follows in which there is little lengthening, and more or less second- 
ary thickening occurs, according to the length of the period during which 
growth of any kind may take place. During the first part there is no evi- 
dence of secretion of rubber in the new parts; during the second, which 
began in 1908 in late July or August, there is a slight evidence of secretive 
activity as regards rubber, though the secretion of resin is synchronous 
with growth. The relation may best be expressed by saying that the secre- 
tion of rubber is a secondary physiological process, its rate of appear- 
ance being inversely to the rate of growth. The rate relation is brought 
out best b}^ plants grown under experimental conditions, in which the 
more rapid growth is accompanied by a less rapid secretion of rubber. No 
exact quantitative statement can be made, since the conditions under 
which experimental plants have been grown have not been fully con- 
trolled. In studying material, I have tabulated numerous observations 
in field and irrigated seedlings, of various ages and at different periods of 
the year, and compared the rubber-content of the cells in all the tissues 
with that in irrigated seedlings. The same has been done for mature field 
and irrigated plants. For this purpose the material which has frequently 
been alluded to was at hand, viz, the branches and stocks of irrigated 
plants at Cedros (plate 4, fig. B) and at Caopas (plate 46, fig. B), both 
immediately at the close of growth-periods and after a period of drought. 
The attempt was made to grade the preparations on the rubber-content 
of the cells, and while this method of procedure has little to recommend 
it for more than approximate accuracy, it enables us to draw reasonable 
conclusions as to the rate of progress of secretion. My observations have 
been digested in the following notes, which will serve to present sufficient 
concrete evidence to support my conclusion. 

1. At the close of the dry season (May 1908) all the cells of rubber- 
bearing tissues produced by growth during 1907, both in new shoots and 
in new tissues in older shoots in field plants, contained rubber in maximum 
quantities (plate 42, fig. 7). 

2. The same may be said, generally, for the field seedlings. There is, 
however, evidence that in the cells of the pith near the top of the seedling 
the maximum content of rubber is not reached. Seedlings (plate 1 7 , fig. 
A) of rapid growth in 1908 had not reached the maximum content (as 
shown both microscopically and by the analysis on p. 187) in April 1909. 
In the cells of the root it was more densely agglomerated than in the stem. 
Here the rubber had the same appearance as in irrigated plants. It was 
only partly agglomerated, and only partially filled the cells. It is quite 
probable that this condition occurs occasionally in mature plants in drier 
habitats after exceptional rainfall and regularly in moister conditions. 

3. A medium-sized twig, grown in 1908, beginning about June i, 
measuring 3.3 mm. in diameter at the base and 1.2 mm. at the tip, was 



184 Guayule. 

examined Aug. 14. At the base the rubber in the pith was finely granu- 
lar, showing in addition a tendency to agglomeration (plate 42, fig. 2) ; in 
the extreme inner and outer cells of the parenchyma rays the rubber was 
very finely granular, while in the cells lying on either side of the cambium 
there w^as none or extremely little; in the primary cortex it was finely 
granular, but was in somewhat larger granules in the secondary cortex; 
large granules occurred in the younger resin-canal cells (in the secondary 
cortex) and agglomerated masses in the older canal cells (in primary 
cortex and pith). Near the apex of the stem the rubber was found only 
in extremely minute granules everywhere (plate 42, fig. i) excepting in the 
resin-canal cells, where they were somewhat larger, but still small (plate 3 1 , 
figs. loto 12). 

4. A similar twig, examined Sept. 8, showed that the condition seen 
at the base in the twig described immediately above had advanced toward 
the apex about one-third the length of the twig. At the base the rubber 
had increased till it had become coarsely granular, except in the paren- 
chyma-ray cells nearer the cambium, in which it was still finely granular. 
Five mm. from the apex there was still scarcely sufficient rubber to be 
observable, except in the resin-canal cells. 

I was unable to obtain material during the succeeding few months, 
so was prevented from following the march of secretion after September 8. 
It is, however, clear that the rate of secretion is so slow, as compared with 
the rate of growth, that for at least four months after the beginning of the 
rainy season the new parts contain onh' very small quantities of rubber. 
From this time on the secretion of rubber probably proceeds more rapidly, 
but it is still to be determined when the maximum is reached. This is a 
point of very great importance. 

5. Turning to irrigated plants, I need cite the evidence from only 
three examinations: 

(a) A branch (plate 2i,figs. A,B)of a Cedros plant (plates 4 and 17, fig. 
B) which began to grow in 1907 and w^as examined in August 1908. In 
examining the 1907 growth no rubber W'as detected in the pith, probably 
because the small amounts secreted in 1907 had disintegrated; the older 
cells (of 1907) in the parenchyma rays contained rubber in fine granules 
near the cortex; in the cortex and resin-canal cells there were coarse 
granules with more or less agglomeration. The new tissues of 1908 con- 
tained only very minute granules. In the 1908 grow^th, near the base, the 
rubber was visible in very fine granules, save in the primary cortex, where 
there was none; in the resin-canal cells coarse granules, these still larger 
in the pith-canals; midway between the base and apex there were very 
fine granules of rubber in the pith and parenchyma rays ; the resin-canal 
cells had coarse or agglomerated granules ; fine granules were visible in the 
secondary cortex, but none in the primary. Four centimeters from the 
apex, where the stem was still herbaceous, minute granules of rubber 
had appeared only in the pith and inner parenchyma-ray cells nearby ; 
it was present in coarse granules in the resin-canal cells of the pith, and 
in fine granules in those of the cortex; the cortex itself contained none 
(plate 43, fig. i). 

{b) A branch from a single Cedros plant collected in April 1909 (plate 
17, fig. B), after a prolonged drought extending with practically no inter- 



The Origin and Occurrence of Rubber. 



185 



ruption from August 1908. Rubber was found in dense rounded agglom- 
erations throughout, but evidently not reaching a maximum content 
(plate 43, fig. 2). 

(c) A branch from a plant grown at Caopas, from stocks transplanted 
by Don Teofilo Delgadillo about January 1908 and taken in October 1909. 
These had less irrigation than the above-mentioned Cedros plants. 1908 
growth: the rubber was densely agglomerated in the whole of the con- 
junctiva (plate 43 , figs. 3 , 4) , in amounts exceeding that in Cedros material 
(plate 43, fig. 2); the 1909 growth contained rubber in coarse granules 
more or less agglomerated throughout. 

6. Irrigated seedlings of all ages up to five months were examined. 
Very young individuals were seen which contained no rubber at all. A 
five-months-old seedling (plate 20, fig. B) contained rubber in coarse 
granules throughout the conjunctiva, being in sufficient quantity in the 
secondary cortex to become agglomerated. 

The method which was used in obtaining the foregoing data, despite 
its limitations, could doubtless be used by the grower of guayule, enabling 
him to follow the behavior of the plants under his charge. The evaluation 
of the evidence is somewhat difficult, but it could be mastered, as may 
be seen, I think, on examining plates 40 to 43. The final control must, 
however, be had by chemical analysis. Tables 52 to 54, which follow, 
contain a few results which comport with the evidence preceding. 

RUBBER-CONTENT BY CHEMICAL METHODS. 

The analysis of the guayule plant in order to determine its rubber and 
resin content presented unexpected difficulties, but the results attained, 
after these had been met, are undoubtedly more reliable than earher 
analyses. I therefore adopt them as exposed in table 52 (Whittelsey, 
1909, pp. 3, 5). 

Table 52. — Percentage of rubber in various parts of guayule shrub. Field plants. 



Parts. 


Rubber. 


Trunk bark 


per cent. 

21.4 

195 

9-7 

0.0 

2.0 


Root bark .... 


Branches and leaves 

Trunk wood 


Root wood 





"The percentage of pure rubber in the whole trunk is 9.9, the whole 
root 7.8, the branches and leaves 9.7, and in the whole plant 9.5, * * * 
based on perfectly dry material. If 'mill weight' is taken as a basis, the 
percentage of pure rubber in the whole plant is 7.8." This result is found 
to correspond very closely to factory experience and the more accurate 
published results, and we may therefore adopt it as exact enough for the 
present purpose. 

The only figures available for irrigated plants are given in table 53 
on the following page. 



186 



Guayule. 



Table 53. — Analysis of irrigated plant two years old from transplanted stocks, Cedros. 
Collected April ^, 1909. Plant weighing /^.^ pounds fresh. 

{1) The original stump planted March 1907, divested of its subsequent growths. 
(II) The growth of 1907 separated into wood and cortex: the wood (Ila), the 
cortex (bark) (116). (Ill) The growths of 1908 intact, and therefore comprising 
both wood and cortex. (IV) The growth of 1909, consisting of short new twigs 
and their leaves, developed before the date of collection. (V) The lateral roots 
intact. 



Number. 


Rubber. 


Number. 


Rubber. 


I 

Ua 

116 


per cent. 

3-55 
0.80 
3.68 


Ill 

IV 

V 


per cent. 

0.67 

3-95 



The method by which the above data were obtained was worked out 
by my former colleague, Dr. Whittelsey. The method was controlled by 
myself microscopically, and the material was found after treatment to 
have been thoroughly, though not quite entirely, extracted. The error 
from this source, as shown by this control, is, however, extremely small, 
and the figures may be accepted as practically correct. 

For the purpose of appreciating the practical significance of the data, 
we may compare the percentage of rubber in the new growth intact. For 
field plants we have a 9.7 per cent rubber-content. In the twigs of the 
irrigated plants studied the amount is 3.3 per cent, namely, a little over 
one-third that of field plants. By comparing Ila and lib, we note that 
this low percentage is due, as shown in Chapter V, to the low percentage 
of rubber in the wood and its relatively larger volume in irrigated plants. 
Moreover, the "branches and twigs" of Whittelsey 's table can not be 
directly compared with those of III in my own, but rather with II and 
III taken together. If it were possible to compare the cortices alone 
we should find, in all probability, a percentage of about 4 per cent of rub- 
ber for irrigated plants against 15 to 20 per cent for field plants, so that 
for the new growths under irrigation from the transplanted stocks in 
question the amount of rubber formed by cortical tissues is about one- 
fourth to one-fifth of that formed in the corresponding tissues in the 
smaller branches and twigs of field plants. But the rate of growth under 
irrigation is such as to result in the production of a volume of cortical 
tissues, at the very least five times greater for the same length of time. 
This factor would be very much increased if field and irrigated seedlings 
were compared. The conclusion would therefore appear to be reached 
that the difficulty attached to the problem of cultivating guayule for the 
rubber is not that of obtaining rubber, but of properly handling the raw 
material so as to extract the rubber from the tissues. 

In the first place, we have repeatedly noted the relatively large vol- 
ume of the wood cylinder in irrigated plants, and its density. We have 
also seen that the branches are long and lithe. If this material is handled 
in its entirety, the volume of barren material which must be handled by 
machinery is considerably greater than in the case of field plants. The 
suggestion (Whittelsey, 1909, p. 6) that the cost of manufacture could 
be reduced by the use of decorticating machinery, as is done in the case 
of "grass rubber" {Funtumia spp.) in x\frica, is still more pertinent for 



The Origin and Occurrence of Rubber. 187 

irrigated shrub, and the character of the growth lends itself to this. This 
would seem to be necessary in the event that the relative amount of rub- 
ber in the cortex can not be raised above 3 . 5 to 4 per cent , not only because 
of this probable difficulty of agglomerating the more finely divided rubber, 
but because of the interference with this of the fragments of splintery 
wood, which will tend materially to obstruct agglomeration in any event. 

In the second place, the individual masses of rubber in the irrigated 
plant are smaller and further away from each other than in field plants. 
Hence, as above said, it is more difficult to agglomerate the rubber. This 
is noted in trying to isolate the rubber from irrigated tissues by mastica- 
tion, a process more difficult than for field plants. It may be found neces- 
sary to introduce a machine especially adapted to mastication of the 
material after passing through the pebble-mill, in which rollers with differ- 
ential speeds will cause the massing of the minute particles of rubber. But 
the practical solution of such problems is not to be obtained merely by 
reasoning about them. The laboratory and factory are mutually of value, 
but the one does not always solve the difficulties of the other. 

VARIATION IN RELATIVE AMOUNT OF RUBBER IN FIELD 

PLANTS. 

I have already pointed out that rubber does not appear in newly 
formed tissues for some time after the cessation of growth ; it may be for a 
period of some months. It therefore appears that the new growth of field 
plants taken at some periods of the year has a content and distribution 
of rubber similar to that in irrigated plants, aside from the relative bulk 
of the tissues themselves. To illustrate, I take the following analvsis of 
seedlings, from Station 2, Quadrat 4 (plate 17, fig. A), collected April 1909, 
germinated in 1908 (table 54). The leaves and stems with tap-roots were 
analyzed separately. 

Table 54. 





Rubber. 


The leaves 


per cent. 
1 .21 
2.40 


The stem and tap-roots .... 



Of interest in this table are the rubber-content of the leaves taken 
separately and the low content of the stems and tap-roots. The leaves 
probably represent the usual condition, as they were old, fully matured 
leaves which had remained attached to the plants throughout a long 
drought period. The plants, however, were of rapid growi;h, indeed 
remarkably rapid for field plants, and the low rubber-content stands in 
relation to this. There is no doubt that this rubber-content is much lower 
than for seedlings of the same size of slow growth. 

In this respect, therefore, there is no hard and fast difference as 
between field and irrigated plants, nor indeed is this the case for the relative 
volumes of the tissues themselves, as I have previously shown (p. 117). 
The response of the guayule under irrigation, therefore, is but an extreme 
expression of what occurs in nature, correlated with the climatic differ- 
ences which obtain from vear to vear, and in different localities. 



1 88 Guayule. 

RELATION OF RUBBER AND RESIN. 

A notion has been widely entertained that the amount of rubber in 
the guayule plant is in some way related to the amount of resin. This 
naturally grew out of the fact that commercial rubbers always contain 
more or less resin, and that resin is abundant in the guayule. In the 
preparation of the commercial article from the guayule the resin becomes 
intermingled with the rubber to the amount of 20 per cent (Whittelsey, 
1909). There appears, however, to be no adequate evidence in support 
of this notion, while on the other hand there is strong evidence to show that 
the physiological processes involved in the secretion of these two materials 
are quite distinct: 

1 . The canals which are laid down in the protogenic tissues become 
actively secreting as regards resin long before rubber appears at all. This 
is strikingly evident in irrigated plants, in which the amount of growth 
is very much in excess of that in field plants. 

2. Resin is secreted in other Compositas in which rubber does not 
occur. In the closely related mariola {Parthenium incanum) resin is 
abundant, while rubber is very meager in amount; and this is true of 
many others. 

3. In irrigated plants the amount of resin is correlated with the ana- 
tomical conditions within the organism, while the secretion proper appears 
to be neither retarded nor advanced by the presence of water. Water, on 
the other hand, affects markedly, though probably indirectly, the rate of 
rubber secretion, Avhich lags behind growth. But the lagging behind of 
rubber secretion is not in inverse relation to any possible increase which 
may be shown to occur in the secretion of resin. 

4. The distribution of starch appears to be connected with the secre- 
tion of resin, as in other well-known instances (e.g.,Pinus). The secretion 
of resin appears, as above pointed out, to be extra-protoplasmic, and in 
harmony with the view expressed by Tschirch, already alluded to. 

5. Rubber, however, appears in the tissues independently of the dis- 
tribution of starch referred to in (4) above. However, the starch found 
in the young tissues near the growing apex may serve as a source of ma- 
terial for the elaboration of rubber. 

6. The appearance of rubber in the canal-cells might be cited to sup- 
port the view^ under discussion, but for the fact that the rubber is merely 
accumulated in these cells and that this occurs later than the secretion 
of resin. Further, rubber occurs in other tissues, e.g., parenchyma rays, 
far removed from resin secretion. Resin in the canal-cells has not been 
demonstrated, but in the meatus only. 

THE SIGNIFICANCE OF RUBBER. 

The inevitable question as to the use of rubber to such a plant as the 
guayule, subject as it is to the severe conditions of the desert, has been 
raised and must be met in some wise. I have already briefly discussed 
the matter (Lloyd, 1909) with but meager satisfaction, as will appear to 
those inclined to find a use for everything in animate nature. I can only 
repeat here what I have already said. 



The Origin and Occurrence of Rubber. 189 

The most obvious suggestion relates to the conser\-ation of water, 
and it seems quite possible that the rubber may act as a sort of blanket, 
reducing to some extent the passage of water to the outer zones of tissue 
and consequently to the outside of the plant, and as a storage material. 
The slower deposition of rubber in irrigated plants and its behavior in 
Castilloa elastica under similar circumstances lend a modicum of support 
to this view. Rubber, as is well known, will take up and retain a certain 
amount of water with considerable tenacity. One would be encouraged 
to hold this view if rapidly grown field seedlings with much less than the 
normal amount of rubber had not been known to pass successfully through 
a long period of drought, indeed much longer than usual. Further, mari- 
ola appears to be as well equipped for resisting drought as guayule, but 
contains a very small amount of rubber. The obvious objection that the 
mariola has some other means to the end would in this case, I believe, be 
difficult to demonstrate, and as difficult to refute. We are here in the 
field of teleological speculation. 

Spence (1908), studying latex, found that this contains oxidases 
capable of acting upon caoutchouc, and argued that this material may 
therefore serve as a reserve food material.^ Similar enzymes probably 
occur in the guayule, but it is safe to remark that in this plant, once the 
rubber is laid down, it is there to stay, as shown by its abjection in com- 
pany with the bark-tissues. Even in the cells adjacent to the active cam- 
bium, or other physiologically active tissues, the amount is never reduced, 
while, if of use as a source of energy to the growing twigs, we should find 
some evidence, analogous to that seen in the starch-content of growing 
twigs, that there is translocation. But such evidence is quite lacking. 
Whatever may prove to be true of latex plants, therefore, there does not 
appear to be the slightest evidence that rubber is in any sense a food 
material in the guayule. 

This view has recently (1909) been again brought into question by 
Spence: 

The fact that the caoutchouc, or rubber, does not occur in any definite latex 
system in the guayule, but in the parenchymatic cells of the medullary rays and 
cortex, and further, that the amount of rubber from the dried plant varies con- 
siderably from one period of the year to the other * * *^ seems at once to 
suggest to my mind that the rubber must have an important function in meta- 
bolic processes. That the rubber is cast off partially and in a modified form in the 
bark, as Professor Lloyd has pointed out, does not in any way weaken the evidence 
of my theory, and from experiments which I have recently made I have found 
that young Ficus elastica trees, grown in an atmosphere and soil free from carbon 
dioxide, gradually drew upon their milk, which became nothing more than water 
after a few weeks' time.- In any case * * * ^he guayule plant shows very 
clearly that we can hardly retain the theory that the latex merely affords protec- 
tion to the plant against internal injury and moisture in time of drought; in 
guayule there is no secretion on injuring the plant, and no reserve water-supply, 
though the rubber is there all the time. * * * " 3 

* See also Cook, 1903. 

' There has been a long controversy on the function of latex, for an account 
of which see Tschirch, 1906. 

^ The quotation was printed in the past tense and third person. I have 
made it into the first person, present. The italics are Spence's. (Lloyd, 1909. Dis- 
cussion, p. 141). 



190 Guayule. 

Dr. Spence adds that sugars are to be found in quantities in certain 
barks, and that the physiological importance of these can not be doubted. 

The answer would seem to be that whatever occurs in Ficus elastica 
can only be of suggestive value with regard to the guayule. And the 
behavior of sugar described means that the unused residue of the sugar 
has been cut out by periderm, just as the unused portion of any other sub- 
stance may be. But this can not mean that everything which appears in 
the bark must have been of use to the plant. The statement made by me 
that the amount of rubber varies from time to time in the year does not 
mean that the absolute amount in a particular individual is now reduced 
and now increased. It means that the amount of rubber relative to the 
weight of the plant is greater at one time than another, and I myself have 
shown this to be the case. The gradual accumulation in the tissues, unac- 
companied by any reduction, of rubber which might serve a storage func- 
tion, this accumulation following growth, seems to completelv contradict 
the view that rubber is a reserve food. We may very well say that during 
growth energy is diverted from the secretion, or, as I should prefer to say, 
excretion, of rubber, and this would accord with the fact that the more 
energy is expended in growth the slower the secretion takes place. 

In the statement to which Dr. Spence refers, when I speak of rubber 
being cast off in a modified form I do not mean to say that this modifi- 
cation is chemical, or that it takes place before the rubber is cast off, 
but by virtue of (presumably) oxidizing processes which take place in the 
cork-cells, which are now dead. This change, it seems to me, can have, in 
the light of the evidence, no significance to the plant. It remains, how- 
ever, to show experimentally that my view is correct, but it can scarcely 
be denied that the evidence against it is tenuous. 

SUMMARY. 

The studies presented in this chapter may be summarized as follows: 

1. In the root, rubber is first secreted in the primary canal-cells (plate 
41, fig. 6), the activity spreading from this region as a center, but more 
rapidly along the radius. At about the same time, or, judging from the 
size of the granules seen, somewhat later, it appears in the innermost cells 
of the parenchyma rays. Rubber appears in the root earlier than in the 
stem in the same plant. 

2. Accumulation usually takes place in the oldest cells first; that is, 
those in the outer zones. Thus, in the root the primary cortex contains, 
before the maximum content for all the cells has been attained, more 
rubber than the cells of the secondary cortex; and the outer cells of the 
latter contain more than the inner. Accumulation (in irrigated plants 
at least) is more rapid in the parenchyma-ray cells than either in the pith 
or the cortex. 

In the primary cortex of the stem rubber may never appear, as, e.g., 
in irrigated plants in which growth and, hence, secondary changes are so 
rapid that the primary cortex does not have time enough for secretion. 

3. With one exception, namely, in the hypocotyl, the accumulation 
of rubber in the stem takes place earlier in the pith than in the paren- 
chyma rays or cortex, and earlier in the rays than in the cortex. 



The Origin and Occurrence of Rubber. 191 

At the apex of the stem of field plants more rubber is found in the 
pith than in the cortex after prolonged drought. 

In the hypocotyl (upper zones) accumulation of rubber takes place 
more rapidly, if not earlier, in the inner parts of the parenchyma rays. 
This appears to be due to a more primitive physiological condition of the 
pith of the hypocotyl. 

4. With questionable exceptions, the accumulation of rubber is 
earlier in the " secreting-cells " of the resin-canals than in the surrounding 
tissues. The exceptions noted were (a) in the apex of a very slowly grown 
field seedling, in the resin-canals of which no rubber was noted, and (6) 
in the new twigs, near the apex of field plants. Rubber may be noted, 
however, in the canal-cells, as in a very rapidly grown irrigated seedling, 
though it occurs nowhere else. 

5. The amount of rubber in the cells of small seedlings ^ in the field is 
relatively as great, or very nearly so, as in mature plants, except in those 
seedlings (table 54) which have grown rapidly in the field, and which 
have not had sufficient time for the accumulation of the full complement 
of rubber. 

6. Rubber occurs unchanged in the portions of the secondary cortex 
which have been more recently cut out by inner periderm. In the cells 
arising directly from the outer or inner periderm rubber does not occur. 
In the bark proper the rubber-bearing tissues alternate with nearly bar- 
ren suber. Volume for volume, therefore, the bark contains less rubber 
than the contingent living cortex which still remains unmodified. 

7. Rubber occurs in the pseudotylose tissue of the resin-canals in 
quantities comparable to the amount found in adjacent cells. 

8. The accumulation of rubber in the new tissues of secondarily 
thickened roots and stems is analogous to that in those still in the primary 
condition. It is for some time absent from the newer parts of the paren- 
chyma rays, and secretion occurs first in the innermost and outermost 
cells simultaneously. The march of the secretion of rubber is, therefore, 
from the base toward the tip of new shoots and from the pith and cortex 
toward the cambium in older stems. 

9. In field plants, that is, in those subjected to the usual desert con- 
ditions of their habitat, the accumulation of rubber is more rapid than in 
irrigated plants. The maximum quantity is certainly not reached in four 
months (June to September, inch, 1908) after growth commences, and it 
is highly probable that six or more months must elapse. 

In a given cell, the amount of rubber in a field plant will generally be 
greater at the end of one year than in a corresponding cell in the irrigated 
plant in two years. Also, cells containing a given quantity of rubber will 
be found nearer the apex of the stem of field plants than of irrigated plants. 
It is probable, again, that the total amount of rubber that a cell in a field 
plant is capable of secreting is greater than in an irrigated plant, though 
this is not certain. 

' Chemical analyses of entire small seedlings are misleading, because of (a) 
the larger relative bulk of the leaves, and (6) the greater relative volume of tissues 
partially filled with rubber, as in the case of .seedlings taken after a period of growth, 
but before the maximum rubber-content has been reached. 



1 92 Guayule. 

The determination of the time at which the maximum rubber con- 
tent is reached is of economic importance, as the earlier gathering of shrub 
involves a considerable economic loss, amounting approximately to the 
quantity of rubber secreted in one year in the new parts. If consistent, 
therefore, with other considerations, the gathering of shrub should not 
occur during, or for some time after the close of, the growing season. It 
will be understood that by new parts is meant the new tissues within the 
already secondarily thickened roots and stems, as well as new accretions 
in length. The time at which the maximum amount of rubber may be 
expected dififers with the length of the growing-season, which depends 
upon the rainfall and the intensity of the drought following. It thus 
happens that field conditions are sometimes such as to produce results in 
field plants (seedlings, table 54) similar to those in irrigated plants. 

10. The rate at which rubber is secreted by irrigated plants, under 
the conditions described for the Cedros experimental plants, is such that 
at the close of the second season's growth (Sept. 1908), the amount in 
the cells is sufficient to agglomerate into the large masses characteristic of 
field plants. This condition was, however, approached after a succeeding 
drought-period lasting till April 1909 (plate 43, figs, i and 2; table 53). 
In plants grown at Caopas under irrigation, during the first season's 
growth (1908) and with a restricted amount of water during the second 
season (1909), the amount of rubber was evidently greater than in the 
Cedros material ' and was great enough by October to agglomerate (plate 
43, figs. 3, 4), forming dense masses, but not as large as in field plants. 
There is, however, a large enough rubber-content in such plants for 
mechanical extraction, though it is probable that some adaptation of the 
process would be necessary. Although the amount of rubber may be as 
low as 3 per cent, it must not be forgotten that the rate of growth under 
irrigation is enormously in excess of that under field conditions. 

1 1 . There appears to be no direct physiological relation between the 
secretion of rubber and of resin. 

12. Rubber appears to have no physiological function in the guayule 
plant. 

' The slowness of secretion in well-watered plants offers an interesting analogy 
to the behavior of the rubber-bearing latex plant Castilloa elastica. (Collins, and 
Pittier; see Cook, 1903). Olsson-Seffer has also pointed out that the secretion of rub- 
ber in this plant is retarded by irrigation, and in consequence it must be deprived 
of water for some time before it can be tapped to advantage. 





^-fMt 




i 



-li IT 'CI 





1. Base of 1908 growth, August. Cedros, Irrigated. 

2. Growth of 1908 in April 1909. Cedros, irrigated. 

3. Cortex, 2-year old stem. October 1909. Caopas, irrigated. 

4. Pith of same plant. 

5. Epidermis of an old leaf, field plant, April 1909. 



CHAPTER Vill. 
VEGETATIVE REPRODUCTION. 

In attempting to solve the problem of the cultivation of a hitherto 
totally feral desert plant, it became necessary to determine quantitatively 
the possibilities of the plant for reproduction vegetatively as well as by 
seed. As has been mentioned, the percentage of germination is small, 
even under the best cultural conditions, so that any haphazard field 
method of sowing seed, in the hope that nature will do the rest, is prac- 
tically out of the question. In the hope that cuttings could be made to 
grow readily and in sufficiently large quantities for cultural purposes, this 
was gone into thoroughly. The net result of all the experiments is to 
show that only a short zone of the stem is capable of root-regeneration, 
nameh^ that immediately above the tap-root, but including some portion, 
difficult to delimit, of the epicotyledonary region in seedlings and an anal- 
ogous portion of the stem in retofios (fig. ii). The ability to produce 
roots in plants from seed is, however, not restricted to the main stem, but, 
as will be shown, resides also in branches springing from the root-producing 
zone. This fact is of rather special biological as well as economic interest, 
and as it throws light on the failure of attempts to grow cuttings I shall 
first present my observations leading to the conclusion stated. 

INDUCED ROOT-REGENERATION. 1 

Both the Mexican guayule (Parthenium argentatum A. Gray) and its 
congener, the mariola (P. incanum H. B. K.), exhibit methods of vegeta- 
tive reproduction which, while shared by other plants, are not common to 
these under the normal conditions of growth. A somewhat detailed ac- 
count of the matter has already been published,^ but a brief restatement 
will be necessary to make clear the point of the present discussion. 

The mariola is a low shrub with rather numerous branches rising 
immediately from the base of the chief stem. These branches arise sub- 
sequently to the development of the chief shoot, and not unusually, during 
the first season of growth, from the seedling. Each following period of 
development sees new lateral shoots of this kind arise again from the base, 
either of the main shoot or, secondarily, from an already well-developed 
basal-lateral shoot. Long continuation of this process results in the dense 
group of stems arising near the surface of the ground which characterizes 
the mature plant of the mariola. 

It is to be further noted that nearly all of these basal-lateral shoots 
are provided with their own root-systems (plate 44, fig. B). From the 
base of each new shoot, soon after it has accomplished a fair amount of 
development, there spring adventitious roots, one of which, by the direc- 

' Presented before Section G of the American Association for the Advance- 
ment of Science at the Baltimore meeting, 1908. 
' Lloyd, 19086. 

13 193 



194 Guayule. 

tion and amount of growth, becomes, in effect, a tap-root of the branch 
from which it springs. Subsequent development of roots of the second 
and higher orders results in the ultimate elaboration of a complete root- 
system. 

We find furthermore that, while the caliber of the basal-lateral stem 
increases with age, the isthmus of tissue between this and the chief stem 
increases only slowly, so that there is never more than a weak connection 
established, and this ultimately becomes disintegrated. In this manner 
the basal branches in question are set free from the parent stock. There 
results, therefore, from a single original stock, a group of independent 
individuals closely crowded together. 

A departure from this behavior is sometimes to be found. A glance 
at the root-system of a single stock will show that the lateral roots run ob- 
liquely into the soil, so that they soon attain a considerable depth. From 
the upper portion of these lateral roots retonos occasionally arise which 
behave much as do the basal-lateral branches above described, and the 
net result is the same, namely, to produce a crowded group of individual 
plants. 

The root-system of the guayule, on the other hand, consists of a 
strong tap-root and several strong laterals, which arise at a short distance 
below the surface of the soil (plate 9, fig. A). These follow a horizontal 
path for a distance, it may be, of 2 meters or more from the plant, and con- 
stitute a water-collecting system by which the plant derives water from 
rain-water which does not penetrate deeply — a feature shared by many 
desert plants (Cannon, 191 1). These shallow roots frequently produce 
root-shoots (retonos) at various distances from the parent stock. I have 
found them at a meter distant, and it is likely that they may arise still 
farther away, though I believe less often than at shorter distances. 

It may be presumed that shoots, arising, as they not infrequently do, 
from the basal portion of the main axis, may occasionally strike root as in 
the mariola. Many thousands of plants, however, have been examined, 
and only one or two cases have been found which may be permitted this 
interpretation. We may therefore regard the method described as the 
only normal method of vegetative reproduction under natural conditions, 
though it has been observed to occur in the field (Station 5) in two cases 
in which the aerial portion of the plant had been removed. 

On observing for the first time the conditions above described in the 
mariola, it occurred to me that it ought to be possible to induce the guayule 
to behave similarly. The fact that a guayule retono strikes new adven- 
titious roots from its basal zone (fig. 11), and that this, in common with 
that part of the chief axis above the cotyledons, has a different ana- 
tomical structure from other stems, gave color to the notion that there 
are physiological grounds for entertaining the belief that, with proper 
treatment, the possibility might be realized. 

As experiments to this end would have necessarily involved a long 
period of time, it was fortunate that I had under observation at Cedros 
plants which had been growing for the major part of two seasons under 
irrigation. This was in September 1908. These plants had grown from 
stumps which were planted in March 1907, by Mr. C. T. Andrews. The 



Vegetative Reproduction. 195 

parent plants had been taken from an old stack-ground in Saltillo, at the 
guayule factory of Martin Brothers, and had started there from seed 
which had fallen from the stacked guayule. Before being transplanted, 
they were variously trimmed back, leaving only the lower portions of the 
main stem and, in some instances, of the lowermost branches. During 
1907 the new growths attained a length of about 25 cm., making rounded 
bushes about 15 cm. in diameter. By September 1908 another 25 cm. 
of growth brought them to a spread of a meter for the largest plants. 

It was then discovered (on the 19th of September) that the lowermost 
new shoots in certain of these plants had struck root, quite after the 
manner described for the mariola, and it was further observed that this 
had not occurred in all of the plants, but either in those plants which had 
been trimmed back so as to leave only a very short basal portion, or in 
those new shoots which had arisen close to the tap-root (plate 44, fig. A). 
In several instances the whole of the lowermost branch was buried by 
chance in the soil, and in others a part, but neither in these nor in some 
layering experiments by Dr. Kirkwood ^ was any response observed. The 
behavior of guayule in this respect is similar to that of certain plants 
which are subjected to mound-layering. Whether it is possible to compel 
every plant properly treated to behave in the manner described can not 
be said, as circumstances prevented a more careful study of the matter.^ 
If this should prove the case, it is evident that the branches which are pro- 
vided with their own root-systems could be removed and transplanted 
with ease. 

PROPAGATION BY CUTTINGS. 

The general conclusion suggested by the above experience was that 
only cuttings taken from the root, or from a portion of the stem near the 
top of the tap-root, would succeed, but as the time of my stay at Cedros 
had drawn to a close it was not possible to direct experiments to test the 
latter of the alternatives. Table 55, summarizing the results of my study 
of cuttings, did not include this particular condition, which could hardly 
have been anticipated. I early found, however, that the stem-cuttings 
made did not respond, and that recourse must be had to cuttings in which 
a portion of root-tissue was involved. The scheme of splitting the butt of 
the plant so as to get two to four pieces was seized upon, the only method 
of those used w^hich secured positive results aside from pure root-cuttings. 

The following conclusions may be drawn from the data in table 55: 

1. Cuttings involving stem-tissues alone, with a possible exception of 
stem-tissue close to the root in seedling or rotono, do not regenerate roots 
under the treatment given. It remains theoretically possible, by special 
and more refined methods, to induce root-regeneration, but for the pur- 
poses toward which the experiments were chiefly directed, this is not 
practicable. 

2. Stem-cuttings may be kept alive, after being planted, for a con- 
siderable period, particularly during the cooler season, by using careful 

' Exp. 181, 182, in which either branches or whole plants were layered. 
^ I have noted the same behavior in guayule from Texas planted by me at the 
Desert Botanical Laboratory and at Auburn, Alabama. 



196 Guayule. 

Table 55. — Experiments in propagation by cuttings. 



Exp. 
No. 


Date. 


Parts used. 


No. 
of cut- 


Conditions. 


Results. 








tings. 








1907 










1-6 


Aug. 


2 


Stem cuttings of vari- 
ous ages. 


65 


Set vertically or hori- 
zontally in trays, 
watered. 


Negative. 


7 


Aug. 


2 


Root cuttings 2 to s 
cm. long. 


25 


Laid horizontally 


Negative. 


8-1 5 


Aug. 


3 


Various parts of 
stem. 


116 


Garden soil, sand and 
soil, manured soil. 


Negative. 


16-17 


Aug. 


3 


Root cuttings from ra- 
pidly grown plants. 


20 


Laid horizontally 


Negative. 


83 


Oct. 


25 


Lateral roots of field 
plants. 


2 


Laid horizontally, 
lightly covered 
with soil (garden 
soil). 


Dec. 3, new shoots i to 
10 mm. long, on one. 
Dec. 24, shoots on 
both. No new roots. 




igo8 








Died later. 


lioa 


Jan. 


24 


Field plants. 1907 
growth pinched off, 
leaves trimmed. 


100 


Planted in limestone 
soil in i-inch paper 
tubes in tray. 
Transplanted Apr. 
6, into prepared 
bed of limestone 
soil, watered and 
shaded. 


Apr. 6, 12 still alive, but 
all died later. No 
roots formed. 


130& 


Jan. 


24 


Ditto, roots 


40 


Ditto 


Mar. 3, 2 buds on one 
cutting. Apr. 6, 9 


















living. 2 more started 














after transplanting. 














Aug. 28, 3 growing 














well. 


130c 


Jan. 


24 


Ditto, 1907 growth 


100 


Ditto 


Apr. 6, 13 alive. All 
died later, no roots 




cut off and leaves 












trimmed off. 






having been formed. 


i3°d 


Jan. 


24 


Ditto, twigs 2 to 3 
years old. 


40 


Ditto 


Apr. 6, 23 alive, but all 
died later. No roots. 








1306 


Jan. 


24 


Ditto, 1907 growth 
pinched off and 


20 


Ditto 


Apr. 6, all dead. No 
roots. 














leaves on left. 








130/ 


Jan. 


24 


Ditto, cut off ob- 
liquely through 


TOO 


Ditto 


Apr. 6, 21 alive. All 
died later. No roots. 










growth of 1906. 














Leaves trimmed 














away. 








131 


Jan. 


27 


Field plants, growth 


40 


Moist sand 


Jan. 21, all dying, rot- 
ting off. 




of 1907 broken off; 












leaves trimmed 














away. 








144 


Feb. 


9 


Twigs 2 to 3 years 


10 


Water 


Apr. 25, all dead. 








old, field plants. 






148a 


Feb. 


24 


"Root-shoot" cut- 
tings; lower part 
of stem and upper 
part of tap-root, 
split into 2 to 4 
pieces. 


31 


Prepared bed^ lime- 
stone soil. 


Apr. 25, growing vigor- 
ously II, starting 8, 
well started but wilt- 
ing, saved by heaping 
soil about them 2. 
Started well but died 
later : ; failed to start 
9. May 2, one more 
started below surface 
of soil; May 19, an- 
other, which later 
died. Aug. 28, 25 to 
32 cm. growth in the 
above living cuttings. 


i486 


Feb. 


24 


Roots 


2 


Ditto 


Both started, one dying 














after making 3 cm. 














growth. 


isa 


Apr. 


4 


Twigs 3 to 5 years old. 
Leaves trimmed 
away. 


25 


Ditto, with shade of 
cloth. 


Apr. 16, all alive. May 
19, 5 budded out. 
May 25, all appear 
dead. May 31. one 
with fresh buds start- 
ing. June 15, all 
dead. 


ISS 


Apr. 


II 


Root-shoot, as in exp. 


50 


Ditto 


After a number had 








148, but from small 






made a start (Apr. 








plants. 






25), all died later. 



Note. — Exp. i to 17 were done jointly with Dr. J. E. Klrkwood. These were preliminary, and not 
under critical conditions. 



Vegetative Reproduction. 



197 





Table 


5 5 . — Experiments 


in propagation by cuttings- 


—Continued. 


Exp. 
No. 


Date. 


Parts used. 


No. 

of cut- 
tings. 


Conditions. 


Results. 


1 60 

i6ia 
161& 
163 
163 


1Q08 
May 19 

May 19 
May 19- 
May 19 
May 19 


Twigs 20 cm. long .. . 

Twigs 15 cm. long, 
leaves not removed. 

Ditto, leaves re- 
moved. 

Twigs IS cm. long, 
foliage trimmed 
away. 

Roots. 2 to 10 mm. 
diameter. 


3S6 

19 
35 
20 
35 


Planted reversed' 
in a "melga," 2 2 
to s cm. projecting 
above surface of 
soil. 

Prepared bed of lime- 
stone soil. Planted 
reversed. 

Ditto 


May 31,3 started; June 
5, 46 started; July 9, 
5S started. In some 
cases buds started 10 
cm. below surface. 
None lived. No roots 
in any case. 

May 29. 5 started, but 
all died later. No 
roots. 

May 29, 4 started, but 
all died later. No 
roots. 

May 26-June 5, 19 
started, all dying 
later. No roots. 

June 10, 7 started, of 
which 6 died. One 
grew well. 


Ditto, not reversed. . . 
Ditto 





1 On the theory that newer tissues might be able to regenerate roots. 

2 A melga is a bed with a deep border to facilitate irrigation by flooding, 
irrigated in this way. 



Alfalfa is frequently 



methods. In many instances they will produce new shoots, the size of 
which varies directly with the volume of the piece. Consequently, exami- 
nation of the above-ground parts might 
easily persuade the uninitiated that 
growth, including that of the roots, had 
taken place. The fact remains that in 
no case had the pieces regenerated roots, 
and in consequence the cuttings all died 
sooner or later. 

3 . Root - cuttings may live and 
become permanently established, but 
under the conditions used the number 
was small (plate 20, A i). In these, too, 
new shoots may be produced without a 
commensurate growth of new roots, and 
the cuttings may therefore die after 
starting. 

4. Sectorial cuttings made by split- 
ting the lower part of the plant in such 
a manner as to involve root and stem 
tissue grow most readily (plate 20, fig. 
A, 2 to 4). The pieces heal completely 
without decaying (fig. 18), and new 
growths of normal extent under irriga- 
tion will be formed, these flowering 
abundantly the first season. Under 
favorable conditions about 75 per cent 
may be expected to live. 

5. Stem -tissue may be forced to regenerate roots by planting the 
basal portions of plants;, trimmed close to the top of the tap-root (plate 
44, fig. A). Branches which then start out will generally behave as the 




Fig. 



18. — A successful sectorial root-stem cut- 
ting, showing complete healing. 



1 98 Guayule. 

basal shoots of mariola do normally, namely, each will send out a root 
from near its base (plate 44, fig. A), which becomes, in effect, a tap-root. 
Thus these branches become established independently of the parent stock, 
and may be separated from it and used for propagation. This occurs in 
nature very exceptionally , but more readily when the top has been removed 
by design or accident ; in the field, however, roots are normally produced 
from the basal portion of the retono chief shoot (fig. 11), from which its 
root-system proper is derived. The more ready production of roots in this 
manner in irrigated plants is connected with the larger supply of water. 

It is seen that the guayule displays a marked polarity analogous to 
that found in plants which will not regenerate roots from stems when ma- 
ture, but will do so when young (Cupressus, vide Goebel, Organography, 
Engl, ed., p. 45, I), and to certain lilies {Hyacinthus sp., Goebel, ibid.), in 
which bulbils are formed from the lower portion of a severed stem, but 
not above. That is to say, the expression of polarity by root-regeneration 
from the stem is definitely restricted to a particular region of certain 
stems only, namely, to the lowermost zone of the branches of the second 
and (probably) higher orders, which themselves arise fiom a narrow zone 
of the chief stem just above the tap-root. 

Shoot-regeneration is, by contrast, easy, and this is true for the root, 
from which stem-primordia are absent. It does not appear that external 
stimulus is necessary, for wounding the cortex of the root in situ is not 
followed, in any of my experiments, by shoot-formation at the point of 
wounding. Nevertheless, as in many plants, a complete severance of a 
root left in situ is frequently followed by shoot-formation, but in a posi- 
tion determined by other conditions, such as dying back, resulting from 
drought. Thus it appears that the notion formulated by Miss Kupfer 
(1907), that the disposition to form roots is much more generalized than 
to form shoots, does not include cases like this before us, which need eluci- 
dation as much as any. And as McCallum (1905) has well said, the prob- 
lem of regeneration is more especially to determine the cause of non- 
development "of parts" in the normal life of the plant. 



CHAPTER IX. 
THE CULTIVATION OF GUAYULE. 

Under the cultivation of guayule must be included all operations 
intended to modify the relation of the plant to its environment. These 
operations may be forestal or cultural, in the narrower sense. It is the 
purpose of this chapter to set forth the conclusions as to the possibilities 
which have presented themselves in both these directions. Although only 
a beginning has been made in the solution of the many difficult practical 
problems which have arisen, the more immediately insistent questions 
involved have been fairly if not completely answered. The difficulty of 
practice is not lessened by the fact that the problem before us is distinctly 
a desert one, and the final answer to many questions may not be obtained 
for many years. 

FORESTAL OPERATIONS. 
PRESENT FIELD OPERATIONS. 

Up to the present time, with only very few experimental exceptions, 
field operations have been confined to the collection of shrub in the great- 
est possible amount with the greatest ease, for the sake of the immediate 
monetary return. This has had both a bad and, in less degree, a good 
result. In many places where shrub had been taken there were so many 
small plants that it was thought that it would not pay to collect them, 
and these will serve to repopulate the areas so treated. In other places, 
where the stand consisted only of large plants, nearly every vestige has 
been removed, leaving at most only the occasional small plants to lay the 
foundations for the future. If in such places a few healthy medium-sized 
plants had been left to produce seed, as common sense should have dic- 
tated, ground that will be barren of guayule for many years might have 
been repopulated, at any rate to some extent. 

The method which has ordinarily been used is to pull up the plant by 
hand, and, while the method of cutting it ofif at the surface of the ground 
has been advocated and to some extent practiced, pulling has been most 
largely used. But in very rocky areas, where the plants frequently grow 
in the fissures of the rock, from which it is often impossible to pull them 
out, the peons will break or twist of¥ the top, leaving the butt in the 
ground. A specific case of this kind was noted by me in a part of the 
Sierra de Ramirez, a range of mountains lying partly in each of the States 
of Zacatecas and Durango, opposite Tan que de la Pendencia. On first 
entering the guayule area, which had been worked in the winter of 1907- 
08, scarcely any guayule was to be seen, but further search discovered 
numerous young growths, visible with difficulty on account of their color 
when seen in April 1909, which had come up from the basal portions of 
plants which had been twisted ofl'. Bare as this ground appeared to be 
of guayule, there is Httle doubt that in time the stand will be replenished 
to a large degree, if not fully. 

199 



200 Guayule. 

In the Lomerio de Zorrillos, some leagues further south, where the 
substratum is made up of fine limestone soil containing stones of various 
sizes, it is easy to pull the plants up, and here all the larger ones were 
taken. As the number of small plants was, however, very great, all these 
were left, and number 600 to 800 to the 100 square meters, weighing i to 2 
tons to the hectare. This condition probably represents the best that may 
occur under the old methods, and is but seldom found. In many spots 
from which larger plants had been taken, pieces of root left by breaking 
off were found to have produced retonos. 

The work of pulling up the guayule is done by peons who tie the shrub 
into bundles, make up burro-loads, and take it to a neighboring " campo de 
guayule," a field-center of operations, where the shrub is baled in hand- 
presses. From here it is hauled in wagons to the most accessible shipping- 
point on the railroad, and so by rail to the factory. 

In undertaking to harvest the shrub from a particular region, the 
usual method is to let a contract to local agents who can command the 
conditions, which, as may well be imagined, are often severe on account 
of the great distances and lack of water. The easiest time to work is 
while the ground is still soft from the rains and when water is relatively 
plentiful, and it happens that this is the worst possible time to take the 
plant as regards its rubber-content. At that time also the shrinkage in 
weight is much greater, both by the loss of a greater amount of water in 
the plant and the larger bulk of the foliage. 

SUGGESTED RULES OF PRACTICE. 

The statement will not need defense that an immediate desideratum 
is a rationale of forestal operation, in order that the present field supply, 
already much reduced from the original stand, may be kept from being 
well-nigh wiped out. The data upon which rules of procedure must be 
based, in the absence of still necessary extensive quantitative study of 
treated areas, have been presented in Chapter IV. The general practice 
indicated by the experiments recorded will therefore be stated here. 

1 . Guayule should be gathered by cutting it off at the level of the 
ground. That which is allowed to project above the surface will die back 
more or less and be an economic loss, as these parts represent a substan- 
tial proportion of the weight of the plants. The cutting should be done 
with a sharp grubbing-hoe {talacho) , a method which is easier on the men, 
as well as contributing to the preservation of the stand of plants. It is 
practically certain that new shoots will arise from many of the parts left 
in the ground, and these, during the first season, will produce flowers, the 
seeds from which will help to repopulate the area. 

It has recently been suggested by Escobar (19 10) that, after cutting, 
a shallow depression be cut in the soil about the remaining root, for the 
purpose of catching the run-off , thus increasing the water-supply. Further 
operations (terracing or furrowing along the contour lines) , designed to 
hold back the run-off, are also recommended. In many situations it 
would be difficult to carry out schemes of this kind. 

2. Only plants 40 cm. or more in height should be taken on the first 
cutting. Five years later there should normally be a crop of 40 cm. plants. 



The Cultivation of Guayide. 201 

which may then be taken. Between 30 and 40 cm. the maximum econo- 
mic efficiency of growth obtains, and this lies between 10 and 15 years of 
age. Fifteen years is therefore the rotation period, but as the growth effi- 
ciency of a plant falls after this age has been reached, these plants must be 
removed each fifth year. The advantage of this rule is to be expected not 
only in the growth of the plants already there, but also in the great effi- 
ciency of seeding. The question has been raised as to the possible increase 
of efficiency of germination by partial or total clearing of the land, thus 
removing the factor of competition. 

3. Removal of the vegetation other than guayule. It is too early to 
make any definite statements as to the value, even with regard to the 
well-being of the mature plant, of clearing operations on guayule lands. 
The experiments which have been initiated involve an area of about 75 
acres, which were well cleared of all vegetation excepting the guayule, the 
"palms" (palma samandoca) which produce fiber, and the few cacti, of 
large species, which occupy little area and do not constitute an aggressive 
element in the vegetation. The clearing of the land has the effect of loos- 
ening the more superficial layers of soil generally, and to some depth in 
spot's. On general grounds this ought (i) to remove competition with 
other plants, which, as has been shown elsewhere, is not insignificant and 
frequently constitutes a real menace to the guayule, necessitating partial 
clearing, at least. This competition of course relates especially to the 
water-content of the soil. Unless the removal of the covering allows 
greater washing than in any event occurs, it should render more water 
available for the guayule and thus enhance growth. It must not be for- 
gotten, however, that a much greater growth is correlated with reduced 
activity in secretion of rubber, either directly or by reducing the volume 
of the rubber-bearing tissues, as has been brought out in the discussion 
of plants under irrigation and from areas of greater rainfall (Chapter V). 
(2) It is important also to know what effect the removal of the vegetation 
has upon the crop of seedlings. The evidence so far obtained appears to 
favor the clearing of the land. I refer especially to the census of seedlings 
made at Station 2 (page 70), in which are recorded numbers of seedlings 
far in advance of those found elsewhere. As to the question of protec- 
tion afforded young seedlings by the shade of other plants, of no small 
importance in many cases, as has been repeatedly indicated by studies at 
the Desert Botanical Laboratory, it may be concluded that the number 
of seedlings which survive a six months' drought, as observed by myself 
in April 1909, is sufficient to warrant the statement that not enough suc- 
cumb to unfavorable conditions to neutralize the good effects, as seen in 
the surviving numbers. It seems probable, therefore, that clearing the 
land of other A^egetation, saving the species above mentioned, is, on the 
whole, beneficial to the guayule. 

As to the specific question of the response in growth, all that can be 
said at this time is guesswork. The areas which were cleared, as it hap- 
pened, were subject to severe droughts from the time of clearing in the 
winter of 1907-08 till the summer of 1909. It is hoped that the abundant 
rainfall of the season now drawing to a close will enable us to form some 
conclusion on this point. 



202 Guayule. 

HARVESTING PERIOD. 

The question of the variation in the relative rubber-content of the 
guayule according to the time of the year is undoubtedly of more impor- 
tance than is at present appreciated. The loss arising from this cause, 
moreover, can not be detected by the chemical control of a factory labora- 
tory, for the reason that the new succulent growths when dried add but 
little to the weight of the plant, while their capacity for rubber-secretion 
is indicated by their living volume. The "shrinkage" between field and 
factory referred to by the manufacturer is equally inefficient as an indi- 
cator of the loss, in a practical sense ; shrinkage consists of all kinds of loss 
in handling and transportation from the field to the factory, an important 
economic factor which, while including the loss under consideration, leaves 
it undiscoverable. 

An element of uncertainty arises from the different moisture-content 
of the shrub at various seasons. Thus, the shrinkage in weight from dry- 
ing in field plants is from 20 to 25 per cent (exactly in my determinations 
between 22 and 23 per cent) during drought; in irrigated plants it is as 
high as 50 per cent. In August 1908, at the height of the growing-season, 
the water-content ranged between 25 and 50 per cent, averaging in the 
neighborhood of 35 to 40 per cent, as high, nearly, as in irrigated plants, 
in which it rarely falls below 40 per cent, and is usually about 50 per cent. 
In addition to this, the weight of the additional leaves in summer is not 
negligible. I shall therefore venture to state with some insistence that, 
assuming normal distribution of rainfall, the gathering of shrub during 
summer months and for several months thereafter can mean, practically, 
only the total loss of the rubber accretion of a whole year. The small 
amounts of rubber undoubtedly present in the newer growths can scarcely 
be recovered by mechanical means, while the ready breakage of the slender 
and weak twigs of recent growth would in any event result in a loss. 

Another consideration is involved also. The germination during the 
growing-season results in the annual crop of young seedlings, the greater 
part of which, on account of the numbers and small size, would undoubt- 
edly be destroyed by the peons at work collecting shrub. Aside from this, 
the peons should be not only instructed but compelled to work carefully, 
so as not to destroy the small plants. 

RESEEDING BARREN GROUND. 

Land from w^hich guayule has been completely removed may, under 
favorable conditions, be restocked by the simple operation of reseeding. 
Whether the cost of doing this would be justified, however, is doubtful, 
since an area of any size would require an immense amount of seed, which 
at present it is difficult to obtain in quantities, and since the percentage 
of germination under natural conditions would be very small. 

Whether the business view will see a sufficient monetary recompense 
in the returns from following the procedure above recommended is not 
the present problem. Local conditions vary too much to solve it in general 
terms. This much, however, may be said: that the rules of operation out- 
lined are dependable in the degree indicated, and that the disregard of 
them, or of some equally or more efficient ones, will only lead to the prac- 
tical extermination of the plant. 



The Cultivation of Guayule. 203 

CULTURAL OPERATIONS. 

Although it will be granted that forestal operations are of immediate 
and great importance for the preservation of the natural stand as a source 
of revenue for as long a period as possible, the ultimate and adequate 
solution of the production of guayule shrub lies in its successful cultiva- 
tion. That this is possible seems at the present not to be overstating the 
case. The abundant and ready growth of guayule under irrigation, its 
drought-resistant qualities, its consequent adaptability to comparatively 
meager irrigation, if this condition is imposed, and its ability to secrete 
rubber, though in relatively smaller quantities per unit-volume of tissue, 
under irrigation properly alternated with drought, are estabhshed facts. 
It remains, therefore, to test, on a larger scale than has hitherto been 
attempted, what may be done to establish the culture of the plant on an 
economic basis. But in stating the positive basis for success the difficulties 
must not be underestimated. These will be indicated in what follows, 
and it will suffice here to point out, in a word, that the great difficulty lies 
in the initial work of establishing the plants, which necessitates water. It 
would be useless to attempt cultural operations without it. 

SEED. 
Should it turn out finally that the raising and the transplanting of 
seedlings is a desirable method of procedure, the obtaining of a sufficient 
amount of seed will be an important desideratum. At present it would 
be necessary to collect seed from the field. This is costly and uncertain. 
Experience has shown that the ripening of seed in the field is uneven ; much 
of it quickly falls off, and the most satisfactory places to collect are fre- 
quently far removed from habitations. Hand-picking is slow, but could 
be rendered more efficient by the arrangement of a device of wire and 
cloth, in two pieces, to be held under the shrub, from which the seed could 
then be dislodged by light beating. It seems, however, unlikely that any 
field method of collecting seed will be as satisfactory as its production by 
irrigated plants, which, in the climate of North Zacatecas, will ripen seed 
for the greater part of the year. The ripening of a large amount at one 
time will render rapid collection easier. Some such device as suggested 
will in any event be necessary, as the seed must be collected and submitted 
to optimum conditions in order to get the maximum germination. It has 
been found that the ordinary conditions of growth, even under irrigation, 
are not efficient for this result. The advantage of growing plants under 
irrigation is in the convenience and economy in obtaining seed, and not 
in its superior quality. Kirkwood (1910a) found that the number of good 
seed from irrigated plants does not exceed 17 per cent, and this is some- 
times, but not often, surpassed in field plants. 

THE RAISING OF SEEDLINGS. 
The small size of the seedlings and their tender character when young 
make it necessary to handle them with considerable care. The precise 
conditions for their successful culture have been studied by Dr. Kirkwood 
(1910a) and by myself, and from these experiences, but more particularly 
from my own, the following practical suggestions are made: 



204 



Guayule. 



The probable necessity of transplanting large numbers of seedlings 
at a very great risk of loss led me to adopt experimentally a scheme used 
in the tropics, where the hollow joints of bamboo are used as flowerpots 
in which to raise cacao, coffee, and other seedlings. When ready for plant- 
ing in the grove the whole pot is planted, and the decay of this, aided by 
fracture, sets the roots free without any disturbance. In a preliminary 
way the joints of "carrizos" {Arundo donax) were tried, but proved too 
small. Combining this idea with that of the paper flowerpot, a unit sys- 
tem of wooden trays and paper tubes was devised,' the tubes being i 
square inch in transverse section and 6 inches long (plate 45, fig. A). As 
trials with these taught that they afforded too little room for the horizon- 
tal development of roots, a comparative test with similar tubes of 4 square 
inches transverse section was carried out under identical conditions. A 
tray 20 by 28 inches inside measure and 6 inches deep was filled with 
these tubes (plate 45, fig. B) , the whole being filled with unsifted limestone 
soil in which there were a great many small fragments of caliche and 
stones. The tray was placed in a melga and watered by subirrigation. The 
surface was shaded at a height of 4 cm. by a thin cotton cloth supported 
in a frame. The shade was raised or lowered as the surface appeared to 
need more or less air, so as to check the growth of fungi (a Coprinus sp. 
was very frequent in the decaying paper of the tubes) , among which one 
species, at least, caused damping-off. On February 16, 1908, 1.5 ounces 
of seed were sown. The germinations were as shown in table 56. 

Table 56. 



Date. 


Germina- 
tions for 
period. 


Total 
germina- 
tions. 


Loss. 


Net total 

living 
seedlings. 


Feb. 26 

27 

28 

29 

30 

Mar. 8 

12 

16 


2 

2 

18 

50 

32 

300 

100 

21 


2 

4 
22 

72 
104 
404 

504 
525 


4 


I 


2 ' 

4 
22 
72 

484 



The appearance of these seedlings is shown in plate 45, figs. C, D, from 
which it will be seen that a good, fairly even stand of sturdy seedlings 
(plate 46, fig. A) was obtained. The size of the tubes used was, of course, 
a compromise, but fig. 19, A, shows that a sufficiently satisfactory root- 
system can grow in them, though of course by no means as good as when 
the roots have normal freedom (fig. 19, B), which in any case is neither 
desirable for practical purposes nor expected. The tray held 140 tubes, 
from which it is seen that there was an average of about 3 seedlings to the 
tube. The unevenness was due to the removal of seed from its original 
position by rain or occasional surface-watering, which is desirable to aid 
in preventing too rapid caking of the surface. To prevent this movement 
of seed the surface should be as level as possible. The margin of the tray 



• By Capt. L. C. Andrews. 



PLATE 45 














1\ 














&!-*:» 



2 u3 



< U 



The Cultivation of Guayule. 



205 



should preferably be somewhat higher than the surface of the soil, as this, 
in addition to enabling one to manage the shade better, prevents the dry- 
ing out of the soil near the edge, in consequence of which the germination 
is not so good. 

The subirrigation may be managed best by placing the trays in melgas 
of a depth sufficient to bring the surface of the water to the level of the top 
of the soil in the tray. In order that the water may gain free access to the 
soil the sides of the trays must be provided with a number of holes. 




Fig. 19. — A, the roots of two seedlings grown in 4-square-inch paper tubes; B, a root-system of about 
the same age, growing in a box. X9/20, 



Despite the apparent indifference of guayule seed to the tem-- -^ratures 
recorded by Klirkwood (1910a) , seeds germinate more pror _ ud, what 
is more important, the seedlings make a much more rapid growth during 
the summer months, as my experiments in July and August 1908 showed. 
In winter, also, the seedlings were frequently killed in part by frost, in part 
by a storm of hail, and were more subject to damping-off. The heavy 
rains of summer also prove more or less destructive, and it was found that 
the seedlings with the shortest hypocotyls survived the best. For this 
reason as thin a shade as possible is desirable, the object of this being to 
preserve the superficial soil-moisture and to cut down the light as little as 
possible. 



206 Guayule. 

When it is desired to transplant the seedlings, the tubes will be found 
to be soft and partially decayed, so that they may be torn by slight pres- 
sure when being placed in the ground. This will favor a prompter adjust- 
ment to the new conditions. The loss will vary and can not be foretold, 




Fig. 20.— The same root-system as shown in fig. 19, B, projected on a 
horizontal plane. 

but with care should be small. As time did not permit an extended trial 
of this, however, I am unable to state economically valuable results, 
though some indication has been had from the following: 

Experiment 79. — Of 449 seedlings (small plants i to 5 years old), 
transplanted into irrigated ground by a peon, 300 lived. The 
transplanting was done Nov. 26, 1907, care being taken to pre- 
vent the roots from drying out, and the ground was well irri- 
gated. On Feb. 16, 1908, the first indications of growth were 
seen, but the plants started unevenly, some not showing signs 
of new growth until Apr. 9. 

Experiment 159. — May 3, 1908, 5 seedlings were transplanted in i- 
inch tubes, the upper 5 cm. of the tube only being preserved. 
All lived and grew well till last observed, Sept. 1908. 

Experiment 153. — Apr. 8, 1908. Of 14 small seedlings (epicotyl 10 
mm. long in the largest) transplanted into a prepared bed, 12 
grew well, 2 died. 

Experiment 164 (J. E. Kirkwood). — On May 13 a bed was prepared 
by digging up the soil and flooding to a depth of 4 inches. On 
the following day i -square-inch paper tubes containing seedlings 
an inch high w^ere set in the wet ground their full length. These 
plants had been grown in the tubes from seed and were some 
two months old. 64 of these were planted, and nearly all lived. 

Experiment 165 (J. E. Kirkwood). — 50 tubes containing plants of 
the same lot as the preceding were set in relatively dry soil 
which showed visible moisture an inch or so below the surface. 
This was done May 1 4 and the ground received water to saturate 
several inches on the i8th and 19th. A few of these survived. 



The Cultivation of Guayule. 207 

Experiment i66 (J. E. Kirkwood). — Bed prepared as above and cov- 
ered with 4 inches of water. On the following day 250 plants 
(seedlings) of three months or less were transplanted into this 
bed. These plants received no more water than what was given 
at the start, in order to test this practice in the transplanting. 
In five days these plants appeared to be dead. 

Experiment 167 (J. E. Kirkwood). — Bed prepared as before and 
watered to saturation. Into this 15 young plants were set on 
May 15 and immediately watered by flooding. The bed was 
watered again on May 16. Nearly all of these lived. It resulted 
that in 164 and 167, in which abundant watering was had at the 
start, nearly all of the plants lived. In the others nearly all 
failed. 

Transplanting cultivated seedlings into the natural habitat was tried, 
but the plants were destroyed by goats. The operation is fraught with 
much difficulty on account of the character of the ground, and would not 
justify itself practically. 

It may properly be said that the raising of guayule seedlings, more 
particularly during the first few weeks, is not a mere rule-of-thumb pro- 
cedure. One has to watch them with care and learn their idiosyncrasies. 
Later they become quite resistant and may be handled much more easily. 

The best soil for them, so far as experiments by Kirkwood (1910a) and 
myself have shown, is the limestone soil of their natural habitat (plate 
16). Soil which contains a good deal of humus appears unfavorable for 
young seedlings, as, among other difficulties, damping-off is very preva- 
lent. However, they were found to have germinated abundantly after 
lying in such soil for seven months, and grew well, though it must not be 
forgotten that the soil must have suffered considerable change by leaching 
and chemical action in the interval. The action of fertilizers has not been 
tried as yet either on seedlings or mature plants. Recent experiments 
have, however, shown that guayule will grow well in a noncalcareous soil, 
and respond readily to sodium nitrate. 

The presence of small stones in the soil appears on the whole to be an 
advantage. The following experiments were done to test this point: 

Experiment 138, Jan. 24, 1908. — Into three root-cages with sloping 
glass sides three lots of seedlings of about equal size were trans- 
planted. One (I) of these contained very finely sifted limestone 
soil; the second (II), similar soil mixed half and half with fine 
angular gravel of limestone; the third (III) was filled with the 
same fine soil and coarse gravel (i cm. ave. diam.), the gravel 
occupying the space that it would without the soil. Feb. 22, 
length of tap-root in (I) 50 mm. ; (II) 50 to 60 mm. ; (III) 60 to 
120 mm. These measurements represent differences in rate of 
growth of the tap-root, which was about the same length in all 
at the beginning of the experiment. 

April 5, the individual measurements of the roots were as follows: 
(I) 10, 10, 10, 10, 12.5, 14, 17 cm. Average, 11. 9 cm. 
(II) 20, 17, 16, 13, 14, 15 cm. Average, 15.8 cm. 
(Ill) 30, 31, 26, 23, 26, 17, 14, 14, 20, 25 cm. Average, 22.6 cm. 

These diflferences were reflected in the aerial development, those of 
III being obviously in advance of the others. 



20 S Guayiile. 

Experiment 139a. — Two seedlings of nearly equal size were planted 
January 24, 1908, in a 5 -gallon oil-can, half of which contained 
a soil made up of coarse gravel and fine soil (of the latter only 
so much as would go between the gravel) , while the other half 
contained uniform, finely-sifted soil of the same kind. The 
watering was equal for both sides, and sufficient to keep an 
abundance of water available. The subsequent growth in the 
plant in gravelly soil was very much more marked, as shown in 
the left-hand plant on plates 18 to 20, the limit of growth for 
the year being nearly reached in four months. This plant, which 
weighed 8 ounces, produced fully 2,000 seeds. The development 
of roots was correspondingly greater in the gravelly soil, and 
careful removal of the roots showed that they were confined 
chiefly to this soil, though occasional roots of each plant reached 
over into the territory of the other. However, it should be noted 
that there appeared to be a tendency of the roots in gravelly soil 
to grow toward the fine soil, as seen in plate 20, fig. A, in which 
the plants are oriented with respect to each other as they grew. 
In these experiments, therefore, the gravelly soil was more favor- 
able to root-development, a result which appears to harmonize 
with agricultural practice. 

IRRIGATION. 

If large numbers of seedlings are to be raised, the method of watering 
will introduce a material element of expense, aside from the cost of the 
water. Hand-watering of the surface would prove to be laborious and 
expensive. For this reason a method of subirrigation was tried, with the 
results as stated above. Additional evidence is as follows: 

Experiment 141. — To test the relative value of subirrigation, with 
and without shade, as compared with surface watering. Four 
trays with i-inch paper tubes (plate 45, fig. A) were filled with 
limestone soil mixed with gravel, each sown with i ounce of seed. 
(I) Placed on the surface of the ground and shaded b}^ a thin 
white muslin screen. 
(II) The same, but without shade. 

(III) Placed in a melga, and shaded as above. 

(IV) The same as III, without shade. 

Ill and IV were watered by subirrigation ; I and II by surf ace water- 
ing, and served as a check on III and IV. It was noted that it 
was very difficult to keep II wet enough. The surface of IV 
was never dry. 

In both shaded trays the germination was far in excess of that in the 
control. In both the subirrigated trays taken together, the germination 
was over twice that in the surface-watered trays, though it was slightly 
more in the shaded, surface-watered tray than in the unshaded, subirri- 
gated tray. The result indicates clearl}' that subirrigation with shade is 
the most favorable of the four conditions. It should be noted that tray III 
was left unshaded after February 13, in order to avoid extreme etiolation, 
and this may have lowered the subsequent rate of germination without 
vitiating the general result. -{ 




u 



The Cultivaiion of Giiayule. 
Table 57. 



209 



Date of count. 



Feb. 7 
8 

9 
10 
II 
12 
13 
14 
r? 
19 
22 
24 
Mar. 16 



Totals 

Less loss to Mar. 16 

Total alive 



Numbers of seedlings in — 



Tray I. I Tray II. 



20 

21 

28 

16 

6 

8 

15 

3 

6 

5 



170 
13 



19 

57 
38 

19 



Tray III. 



18 

34 
42 
58 
40 

25 

13 

8 

IS 



283 

35 

248 



Tray IV. 



9 

6 

3 

25 

10 

1 1 

4 

3 

4 

15 

9 

12 

51 

184 
40 

144 



TRANSPLANTING. 

Another method of getting a stand of gua\mle started and having the 
advantage of speed is by transplanting field plants into irrigated ground. 
Experience has taught that it is of little use to attempt to preserve the 
aerial part of plants of any size, and that even small ones frequently die 
back. Of a plantation of some hundreds of individuals so treated (at 
Caopas), scarcely 25 per cent grew, but upon cutting them back a consid- 
erable additional number revived (plate 46, fig. B). If it should be found 
desirable for any reason to start a crop of guayule from field plants, the 
best method is to cut back to the top of the tap-root and send the tops to 
the factory for extraction. The returns from these would go far toward 
the expense of the operations. It is difficult in any event to start stocks 
unless previously pollarded. 

The portions to be planted should be handled as rapidly as possible, 
being kept from drying out by means of wet burlaps, or some such means. 
They should be planted deeply, the cut surface being no higher than the 
surface of the soil, and they should then be thoroughly irrigated. The 
question as to the amount of water which may be used without doing them 
damage is answered by the simple experiment (exp. 145, Feb. 9, 1908) of 
putting a number of plants into water with their roots and basal part of 
the stem totally submersed. In four days numerous actively growing len- 
ticels were to be seen on the submersed stem, and on March 14 a rootlet 
10 mm. long had grown from one plant, while others had started. By 
February 24 rootlets 6 to 8 mm. long occurred on the upper parts of the 
tap-root, and even roots of the third order were subsequently formed. 
There was no sign of disorganization, so that, unless the soil itself should 
introduce unfavorable elements, we may believe, as indeed experience in 
general shows, that the guayule can stand abundant water. 
14 



210 Guayule. 

The best time of the year for transplanting, as shown by the prompter 
responses of the experiments cited in Chapter VI, is in late spring and 
in summer, when the warmer night-temperatures aid in stimulation. The 
differences in this regard were very noticeable and showed conclusively 
that winter, in North Zacatecas at any rate, is unfavorable for cultural 
operations of any kind. 

The advantage of cutting back to the region of the tap-root, in addi- 
tion to avoiding the loss from dying back, is to be had in the behavior 
which I have described at some length in Chapter VT, namely, the produc- 
tion of basal shoots which root independently. These shoots will be pro- 
duced the more frequently the nearer the tap-root the cut is made. As 
also the guayule frequently sends out new shoots before any new roots 
have been formed, there is less likelihood that these will exhaust the avail- 
able moisture when the whole of the transplanted portion is covered with 
soil. 

HARVESTING CULTIVATED GUAYULE. 

It is almost gratuitous to say anything about this topic, as up to the 
present time the facts have not warranted cultural trials on a scale suffi- 
cient to make available a crop of anything but limited experimental size. 
We are justified, however, in drawing a few conclusions from the facts 
which have been brought to light in the present paper. 

Assuming that the amount of rubber ultimately produced by guayule 
under irrigation is sufficient to warrant its culture, it seems clear that the 
methods of harvesting should be approximately as follows: The new 
growths, say of two years, of plants about a meter in spread,' may with 
advantage be removed by a cutting instrument, so as to leave the butt 
undisturbed to shoot out afresh. The branches which have rooted can 
then be removed by hand simply by breaking them away, and replanted. 
These are usually supplied with a strong root which can be pulled up with- 
out severe damage. In this way the cultivated stand may be increased 
ad libitum, provided areas with sufficient water are at hand. 

CATCH CROPS. 

Immense areas of land are available in the Mesa Central of Mexico, 
and doubtless elsewhere, where "riego temporal" is practiced. This sys- 
tem of irrigation consists of ditches to catch the run-off, leading it to the 
fields. The behavior of guayule would seem to justify the belief that this 
plant could be grown for a sufficient period, say two or three years, in such 
irrigable areas, and the expense, in part at any rate, offset by growing com 
or some other suitable plant, as a catch crop. The guayule, when of suffi- 
cient size, should then be "laid by" to endure a period of drought till it 
becomes usable, when it could be cut as suggested, and restarted. This 
suggestion, and it is that and no more, deserves a serious trial. 

' Assuming the conditions which have constantly been referred to in this work. 



BIBLIOGRAPHY. 

1905. Abbe, Cleveland. The relations between climate and crops. U. S. Dept. 

Agric, Weather Bureau, Bull. 36. 

1906. Altamirano, F. Datos para la historia y explotaci6n del "Guayule." 

Boletin de la Secretaria de Fomento de Mexico (Segunda epoca) 
5: No. lo-i. 1098-1123. This paper is a composite, and contains 
a translation of a paper by G. R. Endlich in Tropenpflanzer 9: 223, 
1905; 11:449,1907; English translation, India Rubber World 1905. 
1908. The Guayule rubber situation. (Unsigned.) India Rubber World 38: 
395.396- The Guayule Rubber Interest, India Rubber World 37: 250. 

1908. Bergen, J. Y. Transpiration of sun leaves and shade leaves of Olea 

europcsa and other broad-leaved evergreens. Bot. Gaz. 38: 285 to 

296. Oct. 20. 
1906. Boodle, L. A. Lignification of phloem in Helianthus. Ann. Bot. 20: 

319 to 321. July. 
1906. Bray, W. L. Distribution and adaptation of the vegetation of Western 

Texas. Univ. Tex. Bull. 82. Nov. 15. 

1909. Butters, F. K. The seeds and seedlings oiCaulophyllumihalictroides. Minn. 

Bot. Studies 4: 11 to 32. June 10. 
1887. Calvert, A., and Boodle, L. A. On laticiferous tissue in the pith of 
Manihot glaziovii and on the presence of nuclei in the tissue. Ann. 
Bot. 1: 55. Aug. 

1905. Cannon, W. A. On the water-conducting systems of some desert plants. 

Bot. Gaz. 39: 397 to 408. June. 

1911. The root habits of desert plants. Cam. Inst. Wash. Pub. 131- 

(Also in Spalding, 1909.) 

1909. Chute, H. O. The deresination of India Rubber, III. India Rubber World 

40: 351 to 352. July. 
1903-4. CoL, A. Recherches sur I'appareil secreteur interne des Composees. 
Jour, de Bot. 17: 252 to 288. Aug.-Sept.; 289 to 318. Oct.-Nov., 
1903. 18: no to 133. Apr., 153 to 175. May, 1904. Contains an 
historical treatment of the subject and an extensive bibliography 
to the date of publication. See also Solereder, Anatomie der Dico- 
tyledonen. 

1900. Cook, O. F. Rubber cultivation for Porto Rico. U. S. Dept. Agric, Div. 

Bot., Cir. No. 28. Contains a brief reference (in foot-note, p. 6) to 
a shrub called "hule," the scientific name of which is not given, 
said to be comminuted and extracted by a physico-chemical process. 
The plant meant is probably guayule. 

1903. The culture of the Central American rubber tree. U. S. Dept. Agric, 

Bureau PI. Ind., Bull. 49. Oct. i. 

1889. Dangeard, p. a. Recherches sur la mode d'union de la tige et de la 
racine. Le Botaniste 1: 75 to 123. 

1906. Dekker, J. De looistoffen. I. Kol. Mus. Haarlem, Bui. 35. Dec. 
1908. Delafond, E. India Rubber World. May. 

1903. Dubard, Marcel. Recherches sur les plantes a bourgeons radifaux. 
Ann. Sci. Nat. Bot. VIII. 17: 109 to 224. 

1910. Escobar, RoMULO. El guayule y su propagacion. Secretaria de Fomento 

Mexico Bol. 24. 
1905. Endlich, G. R. (See under Altamirano, F.) 

191 1. Fox, C. P. Excrement of guayule-fed animals. (Abstract.) Science II. 

23:345- 

1901. Fron et FRAN901S. Le "guayule" plante a caoutchouc du Mexique. 

L'agriculture pratique des pays chauds I: 105 to 109. July-Aug. 
1898. Ganong, W. F. The comparative morphology of the embryo and seedlings 
of the Cactaceas. Ann. Bot. 12: 423 to 474. Dec. 

1907. The organization of the ecological investigation of the physiological 

life histories of plants. Bot. Gaz. 43: 341 to 344. May 16. 

1876. Goldsmith, Sophie. Beitrage zur Entwickelungsgeschichte der Fibro- 

vasalmassen im Stengel und in der Hauptwurzel der Dicotyledonen. 

Dissertation, Zurich. 
1905. Harries, C. Zur Kenntniss der Kautschukarten : Ueber Abbau und 

Constitution des Parakautschuks. Berichte d. D. Chem. Gesellsch. 

38: 1195. 

211 



212 Guayule. 

1906. Hill, T. G. On the seedling structure of certain Piperales. Ann. Bot. 20: 

161 to 175. April. 

1907. Holm, T. Rubiaceae : Anatomical studies of North American representatives 

of Cephalanthus, etc. Bot. Gaz. 43: 153 to 168. 
1908. Medicinal Plants of North America, 22. Enpatormin perfoliaiiim L. 

Merck's Report 17: 326 to 328. 
1909a. Medicinal Plants of North America, 30. Liriodendrontulipiferah. 

Merck's Report 18: 198 to 201. 
19096. Medicinal Plants of North America, 34. Cornus floridah. Merck's 

Report 18: 318 to 320. Dec. 
19 10. Medicinal Plants of North America, 36. Aletris farinosa L. 

Merck's Report 19: 2>2> to 35. Feb. 
1907. HoLTERMANN, C. Dcr Einfluss des Klimas auf den Bau der Pflanzen- 

gewebe. Leipzig. 

1902. JoDiN, H. Recherches anatomiques sur les Boragin^es. Ann. Sci. Nat. 

Bot. VIII. 17: 263 to 346. 

1903. JuMELLE, H. Les plantes a caoutchouc et a gutta. Paris. 

1910a. KiRKWooD, J. E. Propagation of guayule by seeds. Am. Rev. Trop. 
Agric. (Mexico) I: 34-43. Feb.; 77 to 84. Mar.-Apr. 

19106. The growth of guayule in relation to the soil. Am. Rev. Trop. 

Agric. 1: 142 to 159. May-June. 

19 IOC. The life-history of Parthenium argentatum (guayule). Am. Rev. 

Trop. Agric. 1: 193 to 205. July. 

1907. KuPFER, E. Studies in plant regeneration. Mem. Torr. Club 12: 195 to 
241. June 10. 

1906. Livingston, B. E. Soil moisture and evaporation. Cam. Inst. Wash. 
Pub. 52. 

1 90 1. Lloyd, Francis E. Some points in the anatomy of Chrysonta pauci- 
flosculosa. Bull. Torr. Club 28: 446 to 450. 

1905a. The artificial induction of leaf-formation in the ocotillo. Torreya 

5: 175 to 179. Oct. 27. 

19056. — Isolation and the origin of species. Science II. 22: 710 to 712. 

Dec. 1905. 

1908a. The physiology of stomata. Cam. Inst. Wash. Pub. 82. 

19086. Some features of the anatomy of guayule {Parthenium. argentatum. 

Gray). Plant World 11: 172 to 179. Aug. 

1908c. Methods of vegetative reproduction in guayule and mariola. 

Plant World 11: 201 to 208. Sept. 

1908^. A perennial dodder. Plant World 1 1 : 40 to 41. Feb. 

1909. The Mexican Guayule and its Product, pp. 126 to 141 of a volume 

entitled Lectures on India Rtibber: Being the Official Account of the 
Proceedings of the Conference held in Connection with the Inter- 
national Rubber and Allied Trades Exhibition, London, September, 
1908. Edited by D. Spence, Ph.D. London. 

1909a. • Overlapping habitats. Plant World 12: 73 to 78. Apr. 

1910a. The guayule rubber situation. India Rubber World 41: 115 to 

118. Jan. I. 

1 9106. The responses of the guayule, Parthenium. argentatum, to irriga- 
tion. (Abstract) Science II, 31: 434 to 435. March 18. 

1905. MacCallum, W. B. Regeneration in plants. Bot. Gaz. 40: 97 to 120. 

Aug. 16; 241 to 263. Oct. 18. 
1910. MacDougal, D. T. The origin of desert floras. Cam. Inst. Wash. Pub. 1 13: 

pp. 113 to 119. 
1908. Olsson-Seffer, p. Discussion (pp. 21, 22) in the Year-Book of the Rubber 

Planter's Association of Mexico, 1907-1908. Mexico City, Mexico. 

1907. Pearson, H. C. A journey through guayule land. India Rubber World 

35: 173 to 177, Mar. i; 205 to 210, Apr. 1. 

1906. Plowman, Amon B. The comparative anatomy and phylogeny of the 

Cyperaceae. Ann. Bot. 20: i to 33. Jan. 
1899. Ramaley, F. Comparative anatomy of hypocotyl and epicotyl in woody 

plants. Minn. Bot. Studies 2: 87 to 136. Feb. 22. 
1910. Renner, O. Beitrage zur Physik der Transpiration. Flora 100: 451 to 547. 

1908. Ross, H. Der anatomische Bau der mexicanischen Kautschukpfianze 

"Guayule," Parthenium. argentatum Gray. Ber. d. D. Bot. Gesellsch. 
26: 248 to 263. Apr. 23. Contains several pertinent anatomical ref- 
erences. A brief digest in English by A. D. Diisseldorf appeared 
in the India Rubber World 40: 365, Aug. i, 1908, under the title, 
"The anatomical structure of guayule." 



Bibliography. 213 

1909. Sampson, A. W. and Allen, L. M. Influence of physical factors on 

transpiration. Univ. Minn. Bot. Studies 4: ^^. 
1874. ScHWENDENER, S. Das mechanische Princip im Pflanzenreich. Leipzig. 
1909. Servettaz, C. Monographic des E16agnac^es. Thesis (Paris) Dresden. 
1909. Spalding, V. M. The distribution and movements of desert plants. Cam. 

Inst. Wash. Pub. 113. 

1908. Spence, D. On the presence of oxydases in India rubber, with a theory 

in regard to their function in the latex. Biochemical Journal 3: 

No. 4. 
1885. Temme, F. Ueber Schutz und Kernholz — seine Bildung und physio- 

logische Bedeutung. Landwirtsch. Jahrb. 14: 465 to 484. 
1907. Terry, H. L. India rubber and its manufacture. New York. A brief 

reference bearing on the manufacture of guayule rubber, on pp. 55, 56. 
1906. TscHiRCH, A. Die Harze und die Harzbehalter. 2 vols. 1268 pp. Leipsig. 
1908. Die Chemie und Biologic der pfianzlichen Sekrete. 95 pp. Leipsig. 

1909. ToLMAN, C. F. (See Spalding, 1909.) 

1884. Van Tieghem, Ph. Sur la situation de I'appareil s^cr^teur dans la racine 

des Compos^es. Bull. Soc. Bot. France 31: 112 to 116. 

1884. (Remarks on a paper by P. Vuillemin, 1884 a.) Bull. Soc. bot. 

France 31: 1 1 1 , 112. 

1885. Vesque, J. Caracteres des . . . gamopetales . . . Ann. Sci. Nat. Bot. 

VII. 1: 183. 
1884a. Vuillemin, P. Remarques sur la situation de I'appareil s^cr^teur des 

Compos^es. Bull. Soc. Bot. France 31: 108 to no. March 14. 
18846. Note sur le raccord des systemes s^cr^teurs. Bull. Soc. Bot. 

France 31: 266 to 268. May 23. 
1884c. Tige des Compos^es. pp. 258. Paris. 

1909. Whittelsey, Theo. Guayule rubber. I and II. Jour. Ind. and Eng. Chem. 

1: No. 4. Apr. 

1 9 10. WiEGAND, K. M. The relation of hairy and cutinized coverings to transpi- 

ration. Bot. Gaz. 49: 430 to 444. June 23. 
1893. Zimmerman, A. Botanical microtechnique. Transl. by J. E. Humphrey. 
New York. 

Note. — A few of the above citations have been introduced during proof reading for the sake of 
completeness. 



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